# need help in designing an amplifier

Discussion in 'Homework Help' started by blueleni, Dec 25, 2008.

1. ### blueleni Thread Starter New Member

Dec 24, 2008
1
0
Hello guys, I have a circuit design project which is quite challenging.
This project is about designing a 2 stage cascade amplifier and we (our team) are having problems satisfying the given specs.
I hope someone here could shed us some light and save us out of misery T.T

Here are the specs that we should satisfy:

1. Vdd=5V
2. Gain = 60dB
3. Bandwidth = 20MHz
4. LF cutoff = 100kHz
5. Output swing = 4Vp-p
7. Source impedance = 50Ω
8. power comsumption = 5mW

2. ### Wendy Moderator

Mar 24, 2008
21,424
2,951
Are you planning on using BJT's?

3. ### PRS Well-Known Member

Aug 24, 2008
989
36
For what it's worth, here's my thought.

The 20 MHz bandwidth is large. So we need to get this gain (60dB is 1000 volts per volt, right? or is that the power gain?) That is a very large gain at a high frequency which means getting the gain in stages.

Now 20MHZ is quite high and suseptable to oscillations in your amp so you need to use decoupling the supply technique.

Also, you need to use transistors meant for vhf or uhf.

And, as Bill Marden asked, what kind of xistors do you get to use?

The 5mW power consumption defines your load: If the problem is rms power then 4/2=2vpk and this is 2/thesquarerootoftwo:=1.41volts. So 1.14V squared divided by 5mW=228 ohms.

The 4v p-p output is a stinker in that the 2nd stage needs a gain of 4/20mV p-p due to the harmonic distortion restraint. Hence 200 V/V. This means your first stage has a gain of 5 volts per volt.