Side-by-side comparison
| Parameter | Series | Shunt Feedback |
|---|---|---|
| Feedback connection at input | Feedback signal in series with input signal (summed as voltages) | Feedback signal in parallel with input source (summed as currents) |
| Feedback connection at output | Samples output voltage (series-voltage or shunt-voltage) | Samples output current (shunt-current) — combined with input type |
| Effect on input impedance | Increases input impedance by (1 + Aβ) | Decreases input impedance by (1 + Aβ) |
| Effect on output impedance (voltage feedback) | Decreases output impedance by (1 + Aβ) | Decreases output impedance by (1 + Aβ) |
| Gain stabilization | Stabilizes voltage gain A_v = 1/β | Stabilizes transresistance A = V_out / I_in |
| Typical op-amp topology | Non-inverting amplifier (R1, R2 voltage divider to V−) | Inverting amplifier (R_f from output to V−) |
| Example (non-inverting) | LM741: V_out/V_in = 1 + R_f/R1 (series-series) | LM741 inverting: V_out/V_in = −R_f/R_in (shunt-series) |
| Sensitivity to source impedance | Low — high Z_in means little voltage drop at input | High — low Z_in loads the source |
| Best for | Voltage amplification from high-impedance sensors | Current-to-voltage conversion, transimpedance amps |
| Real application | Instrumentation amp front end (INA128) | Photodiode transimpedance amp (OPT101, TIA with TLV2371) |
Key differences
Series feedback at the input sums the feedback signal as a voltage in series with the source — this increases input impedance by exactly (1 + Aβ). In an LM741 non-inverting amplifier with A_v = 100 and open-loop gain of 200,000, the input impedance rises to ~2 GΩ, making source loading negligible. Shunt feedback at the input subtracts the feedback current from the source current — the input node is a virtual ground, and Z_in drops to R_f/(1 + Aβ), often just a few ohms. A transimpedance amplifier (TIA) using a TLV2371 with a 1 MΩ feedback resistor converts photodiode current directly to voltage; the virtual ground at V− keeps the diode bias constant, preventing capacitance nonlinearity. These are not interchangeable: series feedback is for voltage sources, shunt feedback is for current sources.
When to use Series
Use series feedback at the input when the signal source has high output impedance (sensors, electrodes, antennas) and must not be loaded. The INA128 instrumentation amplifier uses series-series feedback internally to achieve input impedance exceeding 10 GΩ, essential for biopotential measurements.
When to use Shunt Feedback
Use shunt feedback at the input when converting a current signal to a voltage — photodiodes, PMT outputs, and current-DAC outputs all need a transimpedance stage. A TLV2371 with a 1 MΩ shunt feedback resistor converts 1 µA of photodiode current to 1 V output with the diode bias held at virtual ground.
Recommendation
The feedback topology is determined by what your source produces and what you want to control. Choose series feedback for voltage-in voltage-out amplification with high input impedance; choose shunt feedback for current-in voltage-out conversion. If you're unsure which applies, look at whether the feedback is connected to sum voltages or currents at the input node.
Exam tip: Examiners ask you to apply the feedback topology table: identify the type (series/shunt at input, series/shunt at output), then write the effect on Z_in and Z_out — most students lose marks by confusing the input and output topology independently.
Interview tip: Interviewers at analog IC design firms ask candidates to explain why a transimpedance amplifier uses shunt feedback and what happens to bandwidth as R_f increases — knowing the gain-bandwidth trade-off (f_−3dB = GBW / A_v) for shunt-feedback TIAs separates strong candidates.