# RF amplifier, why don't two cascaded amplifiers work?

I am currently working on an amplifier circuit for an 8MHz signal (around -40dbm.)

The amplifier is based on an IC chip (LM7171 bin).

The problem is, the amplifier works perfectly individually (correctly gives +20dB gain,) but when we cascade two of them together, it only amplifes +6dB in total.

It is supposed to give +40dB gain. It is probably not a problem of op-amp saturation.

Below is how we cascade our circuit. (VCC=15V, Vee=-15V)

The datasheet is here.

Decoupling capacitor is also connected between the source.

update:

For decoupling, I use 4 capacitors for each amplifier (Vcc=15V, Vee=-15V.)

Both op-amp work well individually (without connecting them together.)

When I meassured the midpoint between two op-amps, it gives +20dB gain. This circuit is on a breadboard.

Update2:

(Please note that this is not the circuit I have now, this is what I had before. Notice that in the picture, the two op-amps are not connected together. I can't take a photo of the current circuit because I don't have them with me. But the only change I make is simply connect them together and change the Rg value a little bit).

• It really sounds like you have a simple wiring error somewhere. Does each of the amplifiers work correctly on its own? Mar 30, 2014 at 16:56
• yes,it does. We simply connect the output of the first one to the input of the second one.
– kuku
Mar 30, 2014 at 16:57
• What sort of power supply decoupling have you used and what sort of construction is the circuit i.e. pcb or breadboard? Also, how have you measured the output signal and if you measured the output signal at the halfway point, does that tally with a single amplifier's gain of 20dB? Mar 30, 2014 at 16:59
• Also check DC bias voltages at input and both outputs. If first amp had >1.5VDC on its output the second output would simply sit at a supply rail, not provide gain.
– user16324
Mar 30, 2014 at 18:11
• If this an example of your general breadboarding technique, it's pretty bad, at least for RF. You have two separate ground busses, but it isn't at all clear how they're connected to each other (if at all). You need to have several direct wires between them, very close to the opamps. Also, what's the idea behind having two of the little blue decoupling caps in series on each opamp power pin? In general, all of your component leads need to be much shorter. Anyway, I'm betting you have a considerable offset bewteen the two ground busses at 8 MHz. Mar 30, 2014 at 18:17