Op-amp Input Modes
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 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
– Ideally, it is zero, since it is the difference between the two input currents.
– In practice:
IOS = |I1 - I2|
– This could become a problem if both dc input currents are not the same.
– Maximum rate of change of output voltage in response to a step input.