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I have a serious doubt about output resistance of transistors. We say that output resistance of a device must be high, for example in BJTs, because it is actually a norton resistance in parallel with the dependent current source of small signal model. But at the same time, we don't want any other device to affect the device we are using in series to it. So, a lower output resistance is required. So, which one of this correct?

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Transistors are used for different purposes, and wired in different configurations to suit those purposes.

As a switch, clabacchio is(was?) correct, we usually want 0 resistance when on, infinite when off.

As a constant current source, (e.g. to drive an LED or charge a NiCd battery) we want infinite dynamic resistance, so the current remains the same as the battery voltage changes.

As a high gain amplifier, we want the output impedance (in parallel with the load impedance) to be as high as possible, so that as the output current varies, we maximise the output voltage variation, i.e. we maximise the gain. This is usually a common-emitter (sometimes common-base) configuration.

As a power output stage, we want the output impedance to be 0 so the output voltage is independent of the load resistance. This is usually accomplished by a common collector aka emitter follower configuration.

This is why high gain amplifiers that must supply high power need several stages, wired in different configurations.

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  • \$\begingroup\$ +1 As an RF output amplifier we want the output impedance to be matched to the load. \$\endgroup\$ – Spehro Pefhany Nov 25 '14 at 12:08
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You are confusing in series resistance and parallel resistance. You want current sources to have high parallel restance and voltage sources to have low series resistance.

BJTs are current sources in the small signal model, so you want as much of an early resistance as you can get.

schematic

simulate this circuit – Schematic created using CircuitLab

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Common collector naturally gives low output impedance, current gain but no voltage gain.

Common emitter naturally gives high/medium output impedance, current gain and voltage gain.

Common base naturally gives high/medium output impedance, voltage gain but no current gain.

You choose the one to suit the performance you require.

enter image description here

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