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 V_out.

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 = A_ol/A_cm

– The higher the CMRR, the better.

– We may also express it in decibels (dB):

CMRR = 20 log(A_ol/A_cm)

– 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 (V_OS)

– Ideally, zero volts out for zero volts in.

– In practice, a small dc voltage, V_OUT(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:

I_BIAS = (I₁ + I₂)/2

Input Offset-Current

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

– In practice:

I­_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.