problem with connecting two stage amplifier

[matin-kh]

Hi every one,
I have a VGA which its gain is varying from -60db to 0db and its bandwidth is 11 GHz.
I have a Post Amplifier(PA) which its gain is about 30db and the bandwidth of it is 9 GHz.
I cascaded this two amplifier which the whole gain would be from -30db to 30db.
my first question is that the bandwidth of the whole circuit would be calculated as below?
(1/(BW)tot)²=(1/VGA)²+(1/PA)²

in the figure below the blue curve is for the VGA when it isn't connected to PA and the red curve is for the VGA when it is connected to the PA I can't understand why the bandwidth is damaged!! what do you think?


in the figure below the blue curve is for the PA when it isn't connected to VGA and the red curve is for the whole circuit the bandwidth is too reduced. what is the problem?

 

[erikl]

Bad matching between the two amplifiers, probably. More reflexion at higher frequencies?

 

[crutschow]

I second erikl's suggestion. At 10GHz, impedance matching between amplifiers is critical for proper signal transfer. How are the two amps connected together?

 

[matin-kh]

there isn't any circuit between them they are connected directly. i think i should use a buffer between them am i right? if so, where i can read more about buffers?

 

[jiripolivka]

In the figures it seems to me are showing some simulations, not real device responses. I do not know any real amplifiers having a flat gain from zero to ...9 GHz.
Your first VGA is not an amplifier but a bare attenuator. Its response can be OK. The second 30-dB amplifier can have the overall frequency response affected by input or output mismatch as shown.
If your VGA1 (0-60 dB attenuator) is a PIN or similar device, then it may be mismatched at the top as well as at the bottom.
A real amplifier with 30 dB gain and 9 GHz BW will also generate a good deal of noise which is not shown. The noise output power can be estimated from Pn = kTB or Pn = -174 + NF + G +10 log BW
where BW is in Hz.
For a good amplifier, NF is say 6 dB, G=30 dB and 10 log BW is ~ 100 dB for 9 GHz. Then, Pn = -174+ 6 +30 +100= -38 dBm, quite a lot.