Block 24 — Wrap-up and Applications
Learning objectives
- connect the different negative-feedback op-amp circuits (Blocks 21–23) into a coherent system view.
- explain how negative feedback determines gain, impedance, and linearity in practical op-amp circuits.
- select an appropriate op-amp circuit (buffer, amplifier, summing, differential, transimpedance, transconductance) for a given application.
- analyze complete signal chains consisting of several op-amp stages.
- recognize practical limitations of real op-amp circuits (supply rails, saturation, loading, offsets).
- interpret op-amp circuits as signal converters (voltage–voltage, current–voltage, voltage–current).
Conceptual overview
- All op-amp circuits in Blocks 21–23 are variations of one single idea:
a high-gain amplifier whose output is fed back in a controlled way.
- Negative feedback forces the differential input voltage $U_{\rm D}$ to become very small, which makes the circuit behavior depend almost entirely on external components, not on the op-amp itself.
- Resistors do not merely “limit current” here — they define signal relationships (ratios, sums, differences).
- Many circuits that look different (buffer, amplifier, converter) are mathematically and conceptually closely related.
- Thinking in terms of signal flow and conversion is the key step from circuit theory to real engineering applications.
Core content
From individual circuits to a system
In Block21, Block22 and Block23, several op-amp circuits were introduced one by one.
At first glance, these circuits may appear unrelated.
However, they can all be understood as special cases of the same feedback principle.
A practical electronic system rarely uses just one op-amp stage. Instead, several stages are cascaded, each fulfilling a specific role:
- Input stage: impedance matching (voltage follower).
- Scaling stage: amplification or attenuation (inverting / non-inverting).
- Combination stage: summing or subtraction (summing / differential amplifier).
- Interface stage: signal conversion (current–voltage or voltage–current).
Understanding why each stage is used is more important than memorizing formulas.
Negative feedback as an engineering tool
Negative feedback provides three essential properties simultaneously:
- Defined gain
The closed-loop gain depends on resistor ratios, not on $A_{\rm D}$.
- Stability and linearity
Small nonlinearities inside the op-amp are strongly suppressed.
- Impedance shaping
High input resistance and low output resistance can be achieved at the system level.
These properties explain why op-amps are ubiquitous in analog electronics.
Typical application patterns
Some recurring patterns appear across many applications:
- Sensor readout
Sensors often deliver currents or small voltages → transimpedance amplifier → voltage amplifier.
- Signal conditioning
Offset removal and scaling → differential amplifier + non-inverting amplifier.
- Summation and mixing
Multiple signals combined with weighting → summing amplifier.
- Actuator drive
Voltage command converted into controlled current → voltage-to-current converter.
Recognizing these patterns allows fast interpretation of unfamiliar circuits.