how the Opamp current gain is determined?

[elecfan]

Can anybody explain for me the no-load and loaded current gain of an amplifier?
the loaded current gain is:

Ai = Vo/Vi , and Vo is the voltage across the load , Rl.

how the no-load gain is determined?

thanks

 

[crutschow]

The no-load or loaded voltage gain is Ai= Vo/Vi where Ai is the voltage gain not current gain. The current gain is only important for certain types of op amp configurations such as constant-current output circuits.

 

[LvW]

Can anybody explain for me the no-load and loaded current gain of an amplifier?
the loaded current gain is:
Ai = Vo/Vi , and Vo is the voltage across the load , Rl.
how the no-load gain is determined?
thanks

The classical opamp in voltage mode has no current gain at all (input and output signals are voltages).
There is another type called "transconductance amplifier" (OTA), that has a voltage-in and current-out characteristic.
But this unit has no current gain, but instead a transconductance G=Iout/Vin.
(But it has no "constant current output" as mentioned by Crutschow).
Is this your question?

 

[elecfan]

The classical opamp in voltage mode has no current gain at all (input and output signals are voltages).
There is another type called "transconductance amplifier" (OTA), that has a voltage-in and current-out characteristic.
But this unit has no current gain, but instead a transconductance G=Iout/Vin.
(But it has no "constant current output" as mentioned by Crutschow).
Is this your question?

to correct my question): as mentioned in the literature, the current gain is calculated as: Ai = Io/Ii.
this is for ideal inverting opamps. while a load of RL is connected to the output. so the output current is as: Io = Vo/RL.

and Ai = Io/Ii = (vo/RL)/(Vi/R1) = Av. (R1/RL) = -R2/RL

This analysis is correct for a loaded opamp, with RL connected.
what about when the opamp is not loaded? can we have an expression similar to voltage gain (as no-load gain for current)?

for the non-inverting opamps the gain is considered to be infinity, since the input current Ii = 0. ?!

thanks

 

[LvW]

OK elecfan,

now I have learned that you speak about an opamp with negative feedback used as an inverting amplifier.
In this case, you can indeed define a current gain Ai=-R2/RL as mentioned above.
But I am not sure if it makes sense.
And it is also true that this definition leads to Ai=0 for a non-inverting configuration.

 

[crutschow]

Why are you concerned about the "current gain" of an inverting op amp voltage amplifier? It has no practical meaning or use. Such a circuit is always represented by the voltage gain (voltage transfer function).

 

[elecfan]

Why are you concerned about the "current gain" of an inverting op amp voltage amplifier? It has no practical meaning or use. Such a circuit is always represented by the voltage gain (voltage transfer function).

it is just to improve my knowledge from the system. I know when we talk about no load gain, practically is not useful.
although I assume that the current gain in loaded condition, should show the attenuation of the current . yes?!

 

[LvW]

although I assume that the current gain in loaded condition, should show the attenuation of the current . yes?!

Do you understand this sentence?

 

[elecfan]

Do you understand this sentence?

well, I think so,
I meant in a voltage amplifier, while the voltage is increased, corresponding to the voltage gain, the current should face attenuation, if the power remains constant.
the current gain should show the level of attenuation. (or amplification)
please let me know if I got it wrong,

 

[LvW]

elecfan,
I really don`t know what you are speaking about (power remains constant? attenuation or amplification?)

 

[elecfan]

elecfan,
I really don`t know what you are speaking about (power remains constant? attenuation or amplification?)

if power gain is one and the voltage amplifies, the current has to get attenuated.
if the voltage increases, the current decreases. (although this is not always the case, for opamps)

if we consider the amplification case, then the current gain should tell you how much the current will get amplified.
I think this should make sense.

 

[crutschow]

The term "Power gain" is usually only used with RF amps, not op amps. The power gain of an amp amp depends upon the input resistance, load resistance and voltage gain. It is zero for no load and goes up inversely with the load resistance. But it is a useless calculation since it is normally of no interest for op amp circuits. You time would be better spend understanding op amp circuits and the normal calculations of their operation, not some useless gain factor.

 

[elecfan]

current gain of an opamp

can someone help me with this theoretical question:

Is current gain of opamp defined just for loaded condition?

In theory, can we talk about no-load current gain?

i.e. Ai = Io/Ii = (Vo/Ro) / (Vi/Ri) = Av . (Ro/Ri)


thanks