# determine the gain of an amplifier

Discussion in 'Electronic Design' started by tiger66, Mar 18, 2007.

1. ### tiger66Guest

Hi All
I am just wondering if I only have the .dc plot of an amplifier to get
the Vin bias point (0.75V) and given the 1mV input signal that's added
to the bias point. Is it possible to determine the gain of the
amplifier?

2. ### Tom BruhnsGuest

Not with only that information. If you also have the circuit, it may
be possible, depending on the accuracy you want and the details of the
circuit.

Cheers,
Tom

3. ### tiger66Guest

So, I have the bias point (0.75V) and the 1 mV input signal voltage
added to the bias point (0.75+0.001)= 0.751V
Then doesn't Vout/Vin = gain?

Or am I complete wrong on this ?

Thanks

4. ### D from BCGuest

No....
In basic design practice, the bias point and signal gain are often
taken as being independent especially for an amplifier that doesn't
distort the signal about the bias point.
However, there are circuits which vary the bias point to vary signal
gain but I'll guess this is not the circuit in question.

D from BC

5. ### Paul Hovnanian P.E.Guest

If you are amplifying a DC signal, yes. Otherwise, the component models
used for determining the DC operating point of an amplifier (treating
capacitors as open circuit, for example) don't give you the correct

6. ### EeyoreGuest

You mean Vout/Vin ?
Yes. Measure the slope of the graph.

Graham

7. ### EeyoreGuest

In fact deltaVout /deltaVin = gain.

Measure the slope of the graph at this point.

Graham

8. ### Tom BruhnsGuest

....
Ah, OK, I didn't understand your original posting...I didn't realize
that you had run (or proposed to run) the DC operating point for two
values of input. In that case, the DC gain between two points (the
input and output) should be clear, as you suggest. That doesn't apply
at other frequencies, however, and will in general change if you put a
load on the amplifier. It also assumes linearity; if the gain is
high, that may not be valid.
analysis. Why not run a .AC or .TRAN analysis? The .AC will easily
give you gain as a function of frequency, assuming linearity about the
determined DC operating point, and the .TRAN can give you gain at
discrete frequencies, and show nonlinearities as well.

Cheers,
Tom