cascading amplifiers

I want cascade three amplifiers which is having 17dB gain. But practically I am getting only 40dB gain, I want full gain. I connected three amplifiers without any coupling. Amplifiers are having inbuilt matching circuit. Please explain where I was lost the gain




Thanking you

Without a circuit we can only guess. You have tested each module on a 50 ohm system? and you are not overloading the output stage when they are cascaded?
Frank

Are you compressing the input of any of the amps? Without specifics, we can only guess a solution.

the inbuilt matching circuit matched to what impedance? theoretically if amplifier matched with 50ohm/same impedance input and output, you can cascade them and get maximum power out..

Hi,

maybe there you reach the limit of rise/falltime.

measure each amplifiers input and the total output. And find out the true gain of each stage.
(I assume the last stage has lower gain.)
As said before: overlaod ... by current.
Or you hit the limit given by the supply voltage.


Klaus

Gains are multiplicative but in dB they will be additive. Theoretically you will be getting 17+17+17=51 dB gain. You are getting about 40dB which is somewhat less.

Please report how much you are getting after the second stage. If you are getting 34dB, the your amplifiers are hitting the glass ceiling (power limited)- no headroom in the final amplifier.

If you are not getting 34dB after the second stage, please measure the gain after the first stage. You will get the answer yourself.

I personally NEVER try to cascade more than around 30 dB gain without some sort of stage to stage shielding. Have had too many problems with self oscillation in the past.

Most amplifiers of RF signals have a limited linearity (P=1 dB) and not-perfect input (S11) and output (S22) matching.
Therefore it is often observed that the gain of cascaded amplifiers is not the sum of their individual gains.

Try to test the cascade with a lower input power to see if linearity is the problem. If you can, try to insert isolators between the stages, or use small coax attenuators with 3- dB loss, to see if matching is the problem.

@biff44, why does oscillation occur in the cases where you have seen it occur?

Last stage is probably saturated or there is an instability in somewhere.
51 dB Gain is not common, ( I don't know the frequency ) and it will create trouble at particularly high frequencies.

well you have say 35 dB of gain, maybe 20 dB of loss of a signal leaking backwards in the enclosure over the tops of your semiconductor devices, and since most systems are broadband, there will be SOME single frequency where you have that big gain AND the round trip phase shift produces constructive feedback--i.e. the conditions to form an oscillator!

"Barkhausen Criteria: For sustained oscillations
1. The total phase shift around a loop is close to 0 degree or 360 degree.
2. The magnitude of the product of open loop gain of the amplifier and the magnitude of the feedback factor is unity.
That is │Aβ│≧1 "