Opreational Amplifiers

What is the role of operational amplifiers?

Elements in our environment such as wind, light, sound, temperature, and humidity change quantitatively on a continual basis. In other words, they are analog values. On the other hand, the processing executed in most devices we use these days is digital. Some form of analog signal processing is therefore required to handle the analog values input to these devices. This analog signal processing is mainly carried out by operational amplifiers.
Let's use a concrete example of analog signal processing wherein the land surface temperature is measured at regular intervals and recorded by a device. First, the land surface temperature is converted into a voltage using a temperature sensor (this is also an analog value). If this voltage is extremely small, there is the risk that the fluctuations cannot be observed, so the voltage must be amplified. This amplification can be done using an operational amplifier (which has the function of obtaining the difference between the signal converted into a voltage and the reference signal, and amplifying this difference through a peripheral circuit of the operational amplifier). Then, the amplified voltage is converted into digital values through A/D conversion, which are then processed by the microcomputer.



As another operational amplifier application example, let's consider an application that measures and records the temperature difference between two points (land surface and 2 m above ground). Likewise, an operational amplifier can be used to obtain the difference (subtraction) between the temperatures measured by two temperature sensors at two points, as follows.



Operational amplifiers also have other functions, such as addition, differentiation, and integration.

In addition to the operational amplifier, a comparator is used to interface (transfer data) to a digital circuit. For the role of a comparator, refer to Comparator Basics.

(2006/03)

What is an operational amplifier?

"Op Amp" is an abbreviation for operational amplifier. Ideally, this is a differential amplifier with the following characteristics:

• Gain: infinite
• Input impedance: infinite
• Output impedance: 0
• Delay from input to output: 0

By negative feedback^Note application, it can be used as an amplifier and for various types of analog signal processing (addition, subtraction, differentiation, integration, etc.).

[Operational amplifier description]
Represented as a triangle, comprises a total of 5 pins, as follows: + input (V_N: non-inverting input => This means that the polarities of the applied input signal and the signal obtained as the output are identical), - input (V_I: inverting input: This means that the polarities of the applied input signal and the signal obtained as the output are exactly the opposite (-180°)), output (V_O), and power supplies (V⁺, V^-).



The most common usage example is that of an amplification circuit, as follows.



In this example, the input signal is amplified by (1 + R₂/R₁) times.
If R₁ = 1 kΩ and R₂ = 10 kΩ, the amplifier will have a gain of x11.
Therefore, the gain can be freely set simply by changing the resistance value. In the above circuit, the signal is being input via the + pin, so the input and output are in-phase (non-inverting amplifier); however, if the signal is input on the R₁ side, the output is inverted (inverting amplifier).

Note Negative feedback means the output signal is fed back to the - input via a resistor, capacitor, diode, or transistor connected between the output and - input.

(2007/03)

What is a single-power-supply operational amplifier?

This is an operational amplifier in which the GND level can be input when a single power supply is used. In other words, V^- is included in the common mode input voltage range.

What are the differences between operational amplifiers and comparators?

There is no strict difference. Generally, however, the range of the common mode input voltage range differs. Products that operate with an input up to the same voltage as V^- are categorized as operational amplifiers for a single power supply. The conventional dual power supply type is a product that cannot be used in a range between V^- and about 1.5 V.

Are there rail-to-rail operational amplifiers whose input and output voltages can be up to the range of the supply voltage?

Unfortunately we do not provide rail-to-rail operational amplifiers.

What is an imaginary short?

An imaginary short is the state where there is no difference between the positive input potential and the negative input potential when negative feedback is applied in an operational amplifier. It is called an imaginary short because the potential between the positive input and negative input is the same in terms of voltage, so it seems as if these pins are shorted.

<Example using inverting amplifier>



In the above diagram, the negative input and positive input have the same GND level. Ideally, the operational amplifier executes amplification operations using feedback characteristics that preserve this imaginary short state.

(2006/03)

What is offset voltage?

In an operational amplifier, the output voltage should theoretically be 0 V when the voltage differential between the input pins is 0 V. However, in reality, V_BE of the positive and negative input transistors in the input stage's differential amplifier circuits differs. As a result, the voltage differential between the input pins is not 0; in fact, a slight amount of voltage is output. This output voltage is known as offset voltage.

Also refer to FAQ opcomp-0002 "What is input offset voltage (V_IO) ?"

(2006/03)

What is offset voltage adjustment via OFFSET NULL pin?

Adjustment is executed in the input stage's differential amplifier circuits to balance any unbalance (caused by a difference in V_BE). Some operational amplifiers have an adjustment pin called the OFFSET NULL pin (see the diagram of the uPC815 below).



The offset voltage is adjusted by externally attaching a variable resistor to this pin to balance the input stage.

(2006/03)

What is the maximum output voltage (Vom)?

This expresses the limit value at which the output of the operational amplifier is saturated when the prescribed power supply voltage is applied. There are two values: one for the positive power supply side and one for the negative power supply side. In addition, either a load resistance value (2 kΩ, 10 kΩ) between the output pin and GND or the output current must be set. The maximum output voltage refers to the dynamic area beyond which the output is saturated in the positive or negative direction, and is limited according to the load resistance value. It may vary depending on the operational amplifier type, but usually the maximum output voltage falls within the range of a voltage 1 V to 1.5 V lower than V⁺ to a voltage 1 V to 1.5 V higher than V^-. Be aware, therefore, that when the power supply voltage (V ^±) changes, the maximum output voltage will also change.

<Vom vs. V^± characteristics curve> Example: Dual general-purpose operational amplifier: uPC251

What is output applied voltage?

This is the range of voltage that can be applied to the output pins without degrading the characteristics or damaging the device.
This rating must never be exceeded, even in the transient state when the power is being turned on and off
Note that the output voltage obtained via an operational amplifier must not exceed the maximum output voltage prescribed in the electrical specifications of the device.

What is common mode rejection (CMR)?

CMR indicates the ratio of the input voltage to the varied input offset voltage when the input offset voltage has changed due to a change in the input voltage.

CMR=ΔV_I/ΔV_IO



Normally the DC variation is the target.
If the CMR value is large, it indicates that, when V_I is the same, V_IO is small.
Therefore, this expresses the extent to which, under identical conditions, mode signals that affect the circuit (noise, drift, etc.) are suppressed with respect to the two inputs.
Normally the DC variation is the target. If the CMR value is large, it indicates that, when VI is the same, V_IO is small. Note that CMR may affect the circuit as an error at the output.

What is supply variation rejection (SVR)?

SVR is the ratio of the power supply voltage to the varied input offset voltage when the input offset voltage has changed due to a change in the power supply voltage.
SVR expresses the extent to which power supply ripple or noise can be suppressed in the device.

SVR=Δ(V⁺-V^-)/ΔV_IO

Example) SVR in the uPC801 is 76 dB (typ). From this it can be calculated that when the SVR is 6.3 × 10³ and the power supply voltage varies temporarily by 0.5 V due to external disturbance, V_IO varies by 79 μV.

What is channel separation?

Channel separation indicates the extent to which the operating operational amplifier affects other operational amplifiers when there are multiple operational amplifiers (2 or 4 channels) incorporated in a package.

What is large signal voltage gain (Av)?

Also known as open voltage (no negative feedback) gain, large signal voltage gain is used in data sheets to show the differential voltage gain in direct current. A value of around 100 dB (x100,000) is standard for an operational amplifier.
The Av vs. f characteristics in the general-purpose operational amplifier uPC151 are shown below. Note how the Av value tends to decrease as the frequency increases.

How many channels' current does the circuit current (I_CC) value indicate?

The circuit current value indicates the total for all channels incorporated in a single package (2 channels: dual type; 4 channels: quad type). Note that this value is calculated assuming there is no load on any channel; in other words, a current that flows through the power supply pins when the output current is 0 A is assumed.

How is the power consumption of an operational amplifier calculated?

The power consumption is calculated from the circuit current of the operational amplifier itself and from the output load current.
Examples of when the output current flows out and flows in are shown below.

(1) Output current flowing out



(2) Output current flowing in





For example, the power consumption in the case of the above diagrams is as follows.

(1) Power consumption from circuit current of operational amplifier itself:
I_CC×(V⁺-V^-)=1(mA)×30(V)=30(mW)

(2) Power consumption from output load current:
I_O×(V⁺-V_O)=10(mA)×10(V)=100(mW)

(1)+(2)=130(mW)

How can I judge the characteristics of an operational amplifier?

You can judge the characteristics of an operational amplifier using the following three parameters.

• Gain-bandwidth product (GBP)
• Zero-cross frequency
• Slew rate

Operational amplifiers in which these ratings are high are faster and have wider bandwidths. However, it is important to choose an operational amplifier based on the frequency of the signals to be processed. If these parameters are not described in the electrical specifications table, use the following three graphs (showing the characteristics curves) to judge them.



This graph shows the relationship between the differential gain and frequency in an operational amplifier. The gain is high in the low-frequency area from DC, and low in the high-frequency area (the shaded area indicates the range permitted when designing a device).



This graph shows the maximum frequency (sine wave) at which a signal can be oscillated without the output being distorted (the shaded area indicates the range permitted when designing a device).



This graph shows the output response characteristics with respect to the pulse input.

What is GBP?

GBP stands for gain-bandwidth product, and is usually calculated at a frequency of 10 kHz. The GBP can be assumed to be within the Aυ vs. f characteristics range.



The shaded area indicates the range permitted when designing a device.
In this example, therefore, it is not possible to design a 60 dB amplifier with a frequency of 100 kHz.

What is the zero-cross frequency?

The zero-cross frequency is the frequency at which the open loop voltage gain becomes 0 dB (×1) and is calculated from the Aυ vs. f characteristics.

What kind of characteristic is the slew rate?

The slew rate (SR) defines the pulse input response. It is a value of the output response of a voltage follower circuit (gain = 1) measured as the voltage amplitude per 1 us. The higher this value is, the faster the performance of the operational amplifier.

What are the output characteristics of the uPC803?

Data of the uPC803 at hand were measured. The results are as shown on the left.
Use this data for your reference.

Test conditions: Room temperature (approx. 25°C)
Supply voltage: ±15 V

Note that this data is a product that happened to be at hand and that the results are not guarantee values.

The V_o vs. I_o (sink) characteristics of the uPC1251 at 15 V are shown. What are the characteristics at 5 V?

The current drops slightly at 5 V but there are no major changes.
Data Sheet (Japanese).

Why does oscillation occur more easily when a capacitor is attached to the load?

Oscillation occurs due to signals being fed back from the output to the input. By positively feeding back an output signal to the input when the gain is ≥1 (0 dB in the graph on the left), the signal amplifies itself and oscillation occurs, even without an external signal. In a negative feedback amplifier, the input and output phases are inverted (by 180 degrees). Therefore, the whole phase is delayed by 360 degrees if the phase of the negative feedback loop is delayed by 180 degrees, making it the same as positive feedback. In this state, if amount of feedback × gain is higher than 1, oscillation inadvertently occurs at this frequency, in the same way as positive feedback. Attaching a capacitor to the output pin causes the phase to be delayed, making it easier for oscillation to occur compared with a resistance load. As shown on the graph on the left, the degree to which oscillation does not occur is indicated by the phase differential of the frequency at which the gain is 1 (0 dB) (known as the phase margin) and by the gain at the point where the phase is 180 degrees (known as the gain margin). In the ideal amplifier, there is no effect from floating capacitance or GND wiring; but in reality, feedback occurs due to these factors. This is why oscillation occurs.

Why is "input resistance" not described in the operational amplifier data sheets?

The input resistance (Ω) can be calculated as R = V/I per unit voltage (1 V) from the input bias current (A) described in the electrical specifications in the data sheets.

An example using the uPC151 is shown

	MIN.	TYP.	MAX.	Unit
Input bias current	-	80	500	nA
Input resistance	2	12.5	-	MΩ

What are voltage noise referred to input (en) and current noise referred to input (in)?

These characteristics are particularly important in audio pre-amplifiers, etc., in order to obtain a high gain from small signals. In the same way as there are two types of offset, two types of noise are generated in operational amplifiers: noise voltage and noise current. The noise that occurs in the input offset voltage is voltage noise referred to input and the noise that occurs in the input bias current is current noise referred to input. Because these values refer to input, they appear amplified at the output in the same way as the input offset voltage. Because the current noise referred to input is multiplied with the signal source resistance, it appears as voltage noise referred to input and is added to the voltage noise. Therefore, the larger the signal source resistance, the larger the noise that appears at the output.

I'm thinking to use a dual operational amplifier. Can you explain what the pairing between each channel (described in the electrical specifications) is?

As a rule, the pairing described in the electrical specifications of a dual operational amplifier is affected by input offset (V and I). Input offset occurs when there is variation in the elements in the input operation stage. The input offset voltage variation is described in the data sheet's electrical specifications.

(2006/03)

When a package has multiple channels (i.e., 4 or 2 channels), does the load capacitance (CL) described in the recommended operating conditions apply to each individual channel?

[Example: uPC842]
Recommended operating conditions

Yes, this value applies per channel when using a package with more than one channel.

(2006/07)

How great can the output current flow of the uPC4558 be?

The output current of the uPC4558 is not specified. Instead, a graph showing the V_O-R_L characteristics is shown on the uPC4558 Data Sheet for your reference. Please calculate the current based on your load resistance (R_L) and corresponding output voltage (V_O) using this graph.

(2006/10)

uPC844: The Japanese data sheet says in the "Cautions on use" that this IC stops sink current flow when the output voltage drops below V^-+0.7 V. What is the reason?

In the equivalent circuit in the data sheet, the output stage transistor (Q20) has an emitter-follower configuration. In order for this circuit to operate, an on-voltage of approximately 0.7 V required for the voltage between the emitter and base in Q20 to operate must be applied. Therefore, as a result, the controllable output voltage range is approximately 0.7 V on up, and the operational amplifier itself cannot output an output voltage lower than 0.7 V.


The OUTPUT SINK CURRENT LIMIT of V_O-I_O SINK characteristics has been included in the data sheet as reference data.

(2006/10)

Is the output current limited if the output of operational amplifier uPC324 is short-circuited?
If short-circuiting is maintained, is quality stress applied?

The uPC324 has an internal output short-circuiting protection circuit. Therefore, the output current is limited as shown below (the following figure is for reference only).

• Short-circuited to V⁺



On the curve of T_A = +25°C in the above figure, I_{O SINK} is limited to about 30 mA.

• Short-circuited to V^- (GND)



On the curve of T_A = +25°C in the above figure, I_{O SOURCE} is limited to about 50 mA.

Note that, if "Total power dissipation: P_T", shown in Absolute Maximum Ratings, is not exceeded even momentarily, quality stress is not applied even if short-circuited status is maintained. Refer to Absolute Maximum Ratings (including Notes 4, 5, and 6).

(2008/04)

Are there any operational amplifiers that can be used with a single power supply?

operational amplifiers can be used with a single power supply. Note, however, that the input and output voltage ranges are limited. The input voltage range changes depending on the type of input transistor.
The usable voltage range is listed as "common mode input voltage range" in the Data Sheet. Check the Data Sheet of each product for this voltage range. Note that the specifications with dual power supply are shown in the Data Sheet.

Can a single-power operational amplifier be used as a dual-power operational amplifier?

Yes. Check the supply voltage range and in-phase input voltage range.

(2006/10)

In a device with multiple channels, how should I handle the operational amplifiers not being used?

The device will not be damaged if you leave the relevant input pins open, but the operation of the open channels will be unstable, which may destabilize the IC internally^Note. It is therefore recommended to make the input pins into voltage followers by connecting the + input to a potential within the common mode input voltage range (e.g. GND).



Note, however, that in the case of the uPC4556, the amplifier must have a gain of at least 10 fold.




The unused pin processing method does not depend on the power supply method. A concrete example is described below.

[Concrete example]
Single power supply operational amplifier: The following power supply methods can be selected when using the uPC324.



Thus, even if the power supply method differs as illustrated above, process the unused pins with a voltage follower (as a recommended connection example). At this time, the + input pin must be set to a potential within the common mode input voltage range.

[Recommended connection example]


Related FAQ

• Operational Amplifiers/Comparators > Commonalities and Differences: The processing methods for the unused pins of unused operational amplifiers and comparators differs. What is the reason?

(2007/03)

I will use a dual operational amplifier (uPC4560), but will not use one of the channels. I plan to connect the input pin of the unused channel to 0 V in V^±= ±15 V, and leave the output pin open.
What kind of problems could occur with such a connection method?

When using this connection method, in which the output is not returned to the input, the gain is extremely large. In this state, even small changes such as in input offset (V_IO) can cause the output to swing sharply to the H level or L level. This V_IO changes depending on changes in the operating ambient temperature (T_A) (the V_IO-T_A characteristic curve of the uPC4560 is shown below). The fact that the output changes as a result of V_IO intersecting 0 V is shown in the following figure. This output change could influence the channel that is used. Please consider using the recommended connection method (voltage follower) for the unused channel.

[V_IO- T_A characteristic]


Related FAQ

• Operational Amplifiers >Application: In a device with multiple channels, how should I handle the operational amplifiers not being used?

(2007/03)

Can an operational amplifier be used as a comparator?

Yes, but note the following points.

• The speed is slower because operational amplifiers incorporate a phase compensation capacitor.
• The output level is not suitable for a digital interface.

The output voltage of an operational amplifier for a single power supply does not fall below 1 V.

The output voltage will not fall below 1 V if a single power supply is used. If an output voltage in the neighborhood of 0 V is required, the characteristics at a low voltage can be improved by inserting a resistor (2 to 5 kΩ) between the output pin and GND for pull-down.
This characteristic is caused by the saturation voltage of the output transistor.
To handle a signal in the vicinity of 0 V, two power supplies, of positive and negative polarities, are necessary.
The minimum output voltage ranges between approximately 0.6 and 1.5 V from the voltage of negative power supply (ground potential in the case of a single power supply).
Note that the output impedance substantially increases at a voltage lower than this.

Does the output voltage fall to 0 V if a single-power supply operational amplifier is used at a common mode input voltage range?

No, it does not fall to 0 V. A voltage of approximately half the supply voltage is output.
If an output voltage of 0 V is required, use an operational amplifier for a dual power supply and set the output voltage so that it is in the center between the positive and negative supply voltages.

Crossover distortion is generated in the output when an audio signal is amplified with the uPC358.

Yes, this is correct.
If the crossover distortion is a problem, use the uPC1458 or the uPC4572 for audio applications, or else use the uPC4742, which does not generate crossover distortion.

The recommended power supply voltage for a low-noise wideband dual operational amplifier is ±4 V to ±16 V. When using a single power supply, can I assume that this recommended value is 4 V to 16 V?
Can I use a single power supply of +5 V?

The power supply voltage range for a single power supply is 8 V to 32 V. You therefore cannot use a single power supply of 5 V.

(2006/03)

Is it OK to connect two operational amplifiers used as voltage followers in parallel?

When two operational amplifiers are connected in parallel, stable operation cannot be expected for the following reason, so such use should be avoided. If this method is used, a difference in potential may exist between output of the two voltage follower operational amplifiers due to the offset voltage (even if the input voltage to the two operational amplifiers is the same). (Different potentials between the two operational amplifiers are connected with a 0 Ω resistor.) Under these conditions, since one operational amplifier outputting higher voltage forces the voltage to rise and the other outputting lower voltage makes it fall, current flows between both outputs, and even if the input voltage changes, it is not tracked and the operation is unstable.

(2006/10)