Main differences between the ideal opamp and a real device:

• Finite gain
  Typical values for low frequencies and small signals: A = 102 ~105

• Finite linear range
  Linear relation v_o = A (v_a - v_b) valid only for a limited range of v_o
  Maximum value of v_o limited by dc supply voltage

• Offset voltage
  Output voltage v_o,off ≠ 0 for v_a = v_b = 0
  v_o,off is usually directly proportional to the gain
  Convenient to describe the offset in terms of the input offset voltage v_in,off
  v_in,off is defined as the differential input voltage needed to restore v_o = 0 in the real device
  for MOS opamps v_in,off ~5-15 mV

• Common-Mode Rejection Ratio (CMRR)
  Differential-mode input voltage:

Common-mode input voltage is defined as:

 

 

Differential gain: A_D = A

Common-mode gain: A_C = v_o / v_in,c

measured as shown in following figure:

 

CMMR = A_D / A_C

Or in logarithmic units:

20 log₁₀ (A_D / A_C) in dB

Common-mode signals undesirables

CMRR measures the degree of suppression of these signals

Requirement: large value of CMRR

Typical values for MOS amplifiers CMRR ~60-80 dB

• Frequency response

Gain A decreases at high frequencies

Evaluation by the frequency f₀ at which |A(f₀)| = 1

Typical values for MOS amplifiers f₀ ~1-10 MHz

• Slew rate

For a large input step voltage, the output follow the input at a slower finite rate

(reason: some transistors may be driven out of their saturation regions)

Slew rate: maximum rate of change dv_o/dt

No direct relation with the frequency response

Typical values for MOS amplifiers ~1-20 V/?s

• Nonzero output resistance

Open-loop output resistance is nonzero, resistive

Typical values for MOS amplifiers with an output buffer ~0.1-5 kW
                                                           with unbuffered output ~ 1MW

Affects the speed of charging a capacitor connected to the output
                                                          thus the highest signal frequency

• Noise

Noise generated by the MOS transistors measured on the output of an opamp, v_on, can be modelled by an equivalent
voltage source, v_n = v_on /A at the opamp input

Typical values of the equivalent input noise source in a wide band (10 Hz to 1 MHz) for MOS amplifiers ~10-50 ?V RMS

Achievable values of the equivalent input noise for bipolar amplifiers  ~3-5 ?V RMS

• Dynamic range

Defined as 20 log ₁₀(v_{in, max} / v_{in, min}) in dB

v_{in, max} ... maximal input signal amplitude in which the device can handle
                                 without an excessive amount of nonlinear distortion

Optimistic estimate: v_{in, max} ~ V_CC /A (+-V_CC ... power supply voltages, A ... open-loop gain)

v_{in, min} ... maximal input signal
      usually v_{in, min} is of the same order of magnitude as the equivalent input noise

Dynamic range of an opamp in open-loop conditions ~ 30 - 40 dB

Dynamic range of an opamp in negative feedback configuration can be much larger

• Power-Supply Rejection Ratio (PSRR)

Incremental component v of the power source gives the corresponding voltage

A_pv on the on the opamp output

PSRR = A_D / A_p (A_D = A ? differential gain)

Or in logarithmic units:
                     20 log₁₀ (A_D / A_p) in dB

Typical values for a single MOS amplifier PSRR ~60-80 dB
                      for a complete filter PSRR ~30-50 dB

• DC Power Dissipation

Typical values for an MOS opamp range from 0.25 to 10 mW dc power drain