Non-Ideal Characteristics

1
Non-Ideal Characteristics

• Input impedance
• Output impedance
• Frequency response
• Slew rate
• Saturation
• Bias current
• Offset voltage

2
Input and Output Impedances

• Ideal model assumes
• RIN is infinite
• ROUT is zero
• In real life
• RIN gt 1 MW
• ROUT lt 100 W

3
Input Impedance

• In either configuration, voltage across RIN will
  be small (ideally zero) if A0 is high. Current
  through RIN should, therefore, be small.
• Effect will be more notable for non-inverting
  configuration where ideal input current is zero.

4
Non-Inverting Amplifier
5
Output Impedance

• To calculate output impedance
• Imagine the input voltage is zero.
• The output voltage should also be zero.
• The output looks like just ROUT connected to
  ground.
• To calculate/measure ROUT, connect a signal
  generator to the output and calculate/measure the
  current.

6
Output Impedance

• With the input set at zero, the equivalent
  circuits for non-inverting and inverting
  configurations are identical.
• Actual output impedance is VOUT/I.

7
Calculating Actual Output Impedance
8
Typically, ROUT appears to be reduced by several
orders of magnitude.
9
Input/Output Impedance Summary

• Negative feedback is very good at compensating
  for non-ideal properties of the amplifier.
• The effects of finite input impedance and
  non-zero output impedance are greatly reduced
  thanks to negative feedback.
• Eg. Using a 741, an amplifier with a gain of 10
  has ROUT of around 100W x 10/105 10 mW!
• NB. Negative feedback will not work so well
  unless the open-loop gain of the op-amp is very
  large.
• Reasonable at d.c. and low frequencies.
• At higher frequencies

10
Frequency Response

• The open-loop gain of an op-amp features in the
  calculations for
• Voltage gain
• Input impedance
• Output impedance
• We assumed it was very large (near infinite)
• True at low frequencies
• Not so at higher frequencies

11
Open-Loop Gain vs. Frequency
12
Effects of Frequency Response
Ideally, gain 10
13
Frequency Response (cont)
Constant, K, depends on the op-amp. For a 741 it
is around 2p106.
i.e. A first order low-pass filter, cut-off
frequency of 100 kHz.
14
Gain-Bandwidth Product

• Cut-off frequency multiplied by mid-band gain is
  always the same value.
• This is the gain-bandwidth product (1 MHz in this
  case).

15
Frequency Response Summary

• It is impossible to design an amplifier whose
  gain exceeds A0(f) at any frequency.
• At high frequencies, gain is limited by A0 which
  typically rolls-off at 20dB-decade.
• The cut-off frequency is
• The intersection of the low and high frequency
  asymptotes
• The 3dB point
• The gain-bandwidth product divided by the
  mid-band gain

16
Slew Rate

• There is a maximum rate of change associated with
  the output of an op-amp. The Slew Rate.
• Typical value for a 741 is 0.5 V/ms.

17
Effect of Slew Rate on a Sine Wave
For a sine wave output voltage of amplitude, A,
and frequency, f
18
Full Power Bandwidth
If the amplitude of the sine wave output is just
below the saturation level, the maximum frequency
that an undistorted SINE WAVE output can be
obtained is often known as the full power
bandwidth. E.g. 741 with saturation levels of
13.5 V NB. More about saturation next time
19
Summary

• Real op-amps deviate from the ideal model in many
  ways.
• Negative feedback automatically compensates for
  many of these.
• Most of the time, therefore, the ideal model
  works pretty well
• except under extreme conditions.
• NB. Saturation comes up next time as an
  introduction to comparators.