Integrated Circuits
Description and operation of logic and linear circuits; introduction to operational amplifiers used as integrator, differentiator, voltage follower, comparator and summing amplifier.
Integrated circuits — summary notes
Free- Integrated circuits are either digital (logic) or analogue (linear). The operational amplifier is the classic linear IC.
- An ideal op-amp has infinite open-loop gain, infinite input impedance and zero output impedance.
- Common configurations: inverting amplifier, non-inverting amplifier, voltage follower (buffer, gain = 1), comparator, integrator, differentiator and summing amplifier.
- Negative feedback returns part of the output out of phase with the input. It reduces gain but improves stability, bandwidth and linearity.
- Positive feedback reinforces the input and causes oscillation — the basis of oscillator circuits.
- ⚠ Exam trap: negative feedback trades gain for stability and bandwidth. A voltage follower has a gain of exactly 1, not the infinite open-loop figure.
- Inverting amplifier gain
- A_v = −R_f / R_in
- Non-inverting amplifier gain
- A_v = 1 + (R_f / R_in)
- Voltage follower
- A_v = 1
An inverting op-amp has R_f = 100 kΩ and R_in = 10 kΩ. What is its voltage gain?
A_v = −R_f/R_in = −100/10 = −10. The magnitude is 10 and the minus sign indicates 180° inversion.
What is the purpose of a voltage follower if its gain is only 1?
Impedance matching. Its very high input impedance avoids loading the previous stage, while its low output impedance can drive the next stage — it buffers without altering the signal level.
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