a simple question about the output impedance and the input impedance

[fightshan]

Is a high output impedance beneficial to the circuit or a low output impedance? and what about the input impedance? is it better for a circuit with a higher input impedance or a lower? It confuses me for a long time, thank you for the reply!

 

[errakeshpillai]

High input impedance is better, so that it will not load the circuit to which it is connected.
Low output impedance is preferred so that the circuit can drive more circuits.

 

[vyomax]

High input impedance is better, so that it will not load the circuit to which it is connected.
Low output impedance is preferred so that the circuit can drive more circuits.

this is specifically related to voltage amplifier.
for current amplifier it is exactly opposite.
for transconductance amp input and output resistance both should be high
for transresistance amp. both should be low


So it depends on application configuration

 

[ftsolutions]

^^^- Yes. It is important to take into consideration what type of circuit and what is considered"better" or "ideal". One wouldn't want a current source to have low resistance, as it defeats the value of having a current source, for example.

 

[fightshan]

^^^- Yes. It is important to take into consideration what type of circuit and what is considered"better" or "ideal". One wouldn't want a current source to have low resistance, as it defeats the value of having a current source, for example.

yes.the question is specifically related to voltage amp,and the most familiar amps are of voltage-voltage type.you told "one wouldn`t want a current source to have low resistance",is the reason that low impedance will overload the voltage source?

 

[ftsolutions]

you did not specify in your original query that this was about voltage amplifier circuit, though we have assumed that until now. But even internal to a voltage amplifier designs (which in any but the most simplistic) will have constant current source(s) to improve power supply noise rejection, and to provide a good, consistent load for the Voltage amplification stage (VAS), among other things. Once you get beyond a simple 1-transistor circuit, you can see that more complex circuits such as amplifier topologies consist of numerous subcircuits, each of which need to have specific input/output impedance characteristics to do their job well, and the the proper operation of the whole device relies upon them all.

 

[Borber]

RF amplifiers has equal input and output impedance. Both are low let's say 50ohms.