Op-Amp Input Modes and Parameters



Op-Amp Input Modes and Parameters

There are several input modes for an op-amp.

i) Single-Ended Input.

One input is grounded and the input voltage is applied to the other.

If the voltage is applied to the inverting input, an inverted amplified signal is seen at the output.

A non-inverted amplified signal is seen if the input voltage is applied to the non-inverting input.

ii) Differential input.

Two opposite polarity (out-of-phase) signals are applied to the inputs.

The amplified difference between the two inputs appears on the output.

iii) Common-Mode Input.

Two signal voltages of the same phase, frequency, and amplitude are applied to the two inputs.

They should cancel out to yield an output equal to zero.

This is called the common-mode rejection.

Its importance lies in the situation where an unwanted signal appears commonly on both op-amp inputs.

The unwanted signal will not appear at the output.

Thus, only the desired signal will show up at Vout.

Common-Mode Rejection Ratio (CMRR)

This is the measure of an amplifier’s ability to reject common-mode signals.

Ideally, complete rejection of common signals should be seen.

However, there is some small common-mode gain (much less than 1).

The higher the open-loop gain with respect to the common-mode gain, the better the performance of the op-amp (in terms of rejecting unwanted signals).

Thus, the ratio between the open-loop voltage gain and the common-mode gain becomes:

CMRR = Aol/Acm

The higher the CMRR, the better.

We may also express it in decibels (dB):

CMRR = 20 log(Aol/Acm)

The open-loop voltage gain (also called the large-signal voltage gain) is the gain of the device when there are no external components.

It may be up to 200000 and it is not a well controlled parameter.

A CMRR of 100000 means that the desired input signal (differential) is amplified 100000 times more than the unwanted noise (common-mode).

Common-Mode Input Voltage Range

Range of input voltages which, when applied to both inputs, will not cause clipping or other output distortion.

Typical values ±10 V with dc supply voltage of ±10 V.

Input Offset Voltage (VOS)

Ideally, zero volts out for zero volts in.

In practice, a small dc voltage, VOUT(error), appears at the output with no differential input voltage applied.

Typical values are between 2 mV or less.

Input Offset Voltage Drift with Temperature

How much change there is in the input offset voltage for each degree of change in temperature.

Typical values are between 5 ?V/oC to 50 ?V/oC.

Input-Bias Current

Input bias current is the dc current required by the inputs of the amplifier to properly operate it.

The input bias current is the average of both input currents:

IBIAS = (I1 + I2)/2

Input Offset-Current

Ideally, it is zero, since it is the difference between the two input currents.

In practice:

OS = |I1 - I2|

This could become a problem if both dc input currents are not the same.

Slew Rate

Maximum rate of change of output voltage in response to a step input.

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