While measuring the output impedance of an amplifier, the input is replaced by the internal impedance of the source because in this measurement :
no signal should be transferred from input to output
Output impedance is defined with the input signal switched off. Formally,
\(Z_{out}=\left.\dfrac{V_{o}}{I_{o}}\right|_{V_{S}=0}\)
— a test signal is applied at the output terminals and the resulting current measured, while the input source is deactivated. "Deactivated" for a voltage source means replaced by a short circuit, but a practical source is a voltage source in series with its internal impedance, and only the ideal part may be removed. What remains is the internal impedance itself, which is exactly what the question describes.
Why the input source must be silenced. If the source kept driving the amplifier, its signal would appear at the output added to the test signal, and the measured ratio would be meaningless — the reading would depend on the amplifier's gain rather than on its output impedance. Killing the source guarantees that no signal is transferred from input to output, so whatever appears at the output is purely the response to the test signal. That is option 1.
| Measuring | Test signal at | Other port |
|---|---|---|
| Input impedance | Input | Output loaded with its normal RL |
| Output impedance | Output | Input source killed, internal Z left in place |
Why the other options fail. Impedance matching (option 2) is a design objective for power transfer, not a condition for a measurement, and forcing a match would change the very quantity being measured. Option 3 states a property of the amplifier rather than a reason for the procedure — and in any case the amplifier's input impedance is left exactly as it is; it is the source that is altered.
The practical version of the same idea. A common bench method avoids an output generator altogether: measure the open-circuit output voltage \(V_{oc}\), then connect a known load RL and measure \(V_{L}\). Then
\(Z_{out}=R_{L}\left(\dfrac{V_{oc}}{V_{L}}-1\right)\)
Here the source is driving, but the method works because the same input signal is present in both readings and cancels in the ratio — a reminder that the requirement is really that the input contribute nothing new between the two measurements.
The underlying principle is Thevenin's theorem: to find the equivalent impedance looking into a port, all independent sources are set to zero while their internal impedances stay in circuit. Dependent sources, by contrast, are always left active — which is why an amplifier's output impedance can be far lower than any physical resistor in the circuit.
Hence, the input is replaced by the source impedance so that no signal is transferred from input to output.
Similar Questions
From the following statements :
(A) IDSS is the maximum drain current for a JFET with a open circuit connection from gate to source and VDS > VP (Pinch off voltage)
(B) The magnitude gain corresponding to a voltage gain of 100 dB is 1,00,000
(C) For any inverting amplifier, the input capacitance will be increased by a Miller effect capacitance sensitive to gain of amplifier and feedback capacitance (Cf) between input and output terminals of active device
(D) The high frequency transistor small signal AC equivalent circuit model i.e. Hybrid-π is known as Giacoletto model
(E) The transistor bias configuration having high stability factor are stable and relatively insensitive to temperature variationsChoose the most appropriate answer from the options given below :
Match the following :
List - I List - II (a) A/D Convertor (i) Impedance Matching (b) Emitter Follower (ii) VCO (c) Schmitt Trigger (iii) Successive Approximation (d) Phase Locked Loop (iv) Hysteresis Codes :
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