# output impedance and input impedance of an amplifier

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#### ajetivarg

##### Newbie level 4
Why we need to short the input source when we calculate the output impedance???
Why we need to short the output source when we caculate the input impedance??

I have seen some threads in which they explain using thevenin theorem, i am not satisfied with that can anyone give some alternative explanation which is more clear.

#### Audioguru

The output of an amplifier has nothing to do with its input impedance. Then shorting the output will not affect the input impedance.

The output impedance of an amplifier can be measured by feeding an input signal and loading it until the output signal voltage drops to half (maybe with lots of smoke).
Or it can be measured by feeding a signal into its output through a very low resistance and adjusting the resistance until the signal at the amplifier drops to half.
The input is not shorted with both ways.

ajetivarg

### ajetivarg

points: 2

#### The Electrician

##### Full Member level 5
Why we need to short the input source when we calculate the output impedance???
Why we need to short the output source when we caculate the input impedance??

I have seen some threads in which they explain using thevenin theorem, i am not satisfied with that can anyone give some alternative explanation which is more clear.

Provide a link to those threads (at least one, please), so we can see what kind of amplifier is being discussed, and what assumptions are being made.

#### Dan Mills

Audioguru, what you say might be true of a typical audio amp, but it is a long way from true in many RF amplifiers where S21 is anything but zero.

Consider a norton transformer feedback amplifier (Which in typical RF usuage is a common base stage with transformer feedback between the collector and base circuits), its input impedance is very strongly related to the load impedance and it has markedly poor isolation (But some other good properties).

We need to know what kind of amplifier is being discussed and in what context before it is possible to really comment intelegently.

Regards, Dan.

#### ajetivarg

##### Newbie level 4
The output of an amplifier has nothing to do with its input impedance. Then shorting the output will not affect the input impedance.

The output impedance of an amplifier can be measured by feeding an input signal and loading it until the output signal voltage drops to half (maybe with lots of smoke).
Or it can be measured by feeding a signal into its output through a very low resistance and adjusting the resistance until the signal at the amplifier drops to half.
The input is not shorted with both ways.
HI audioguru,

Sorry, i didn't understood what u said above. I am refering to common gate amplifier.

regards,
ravi

#### Audioguru

A common gate amplifier is very different from an opamp or a common source amplifier.

#### FvM

##### Super Moderator
Staff member
In general terms, we can assume that an amplifier has a certain forward and backward transmission, in so far the load of one port affects the impedance of the other. The effect may be negligible for some amplifiers, e.g. low and medium frequency OPs.

There are different ways to specify a linear two-port. http://en.wikipedia.org/wiki/Two-port_network

If you refer to impedance parameters (Z-parameters), the other port would be connected open circuit, not shorted, according to the nature of the parameter set.

ajetivarg and LvW

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### ajetivarg

points: 2

#### an_82

##### Member level 2
Dear ajetivarg,

Design of Analog CMOS Integrated Circuits, Behzad Razavi

ajetivarg

### ajetivarg

points: 2

#### Borber

ajetivarg is talking about y11 and y22 and mixed it with input and output impedances of an amplifier.

#### LvW

The output of an amplifier has nothing to do with its input impedance. Then shorting the output will not affect the input impedance.
I don`t think that is correct.
More than that, it is just a matter of definition.
As a simple example:
Two of the h-parameters describing the small signal behavior of a BJT are defined for such short circuit conditions (small-signal input resistance (hie) and current gain (hfe) are defined for ac shorting the output.

EDIT: More than that, for the majority of all feedback amplifiers it is absolutely necessary to short the input (ac) for determining the output impedance. Otherwise, the results are false because they do not reflect the actual operating conditions..

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#### ajetivarg

##### Newbie level 4
In general terms, we can assume that an amplifier has a certain forward and backward transmission, in so far the load of one port affects the impedance of the other. The effect may be negligible for some amplifiers, e.g. low and medium frequency OPs.

There are different ways to specify a linear two-port. http://en.wikipedia.org/wiki/Two-port_network

If you refer to impedance parameters (Z-parameters), the other port would be connected open circuit, not shorted, according to the nature of the parameter set.
Hi FvM,

Thanks for the reply. So, what you are trying to say is that we are making an approximation when we are calculating the input and output impedance which is true in most cases. Is it what you are trying to say???
By the way, when i am say shorting the input source in the question, i mean shorting the input voltage source. If the input source is a current source what you said might be true.

regards,
ajetivarg

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