Super High Input Impedence with Low Signal, to a High Vo over a low Impedance

Thread Starter


Joined Apr 22, 2009
I have been trying for a long time to figure this one out. Attached are some of my attempts.

Figure 1 is what my professor has given me as constants/cannot change (other than a reasonable freq; we are not too interested in extremes). We need to do it twice: one with BJT's and one with FET's. We cannot interchange the parts (like have an fet/bjt combo in one circuit) and we cannot use op amps.

If the picture is ambiguous, here are the requirements in words:

-5V (pp) input straight into a 100kΩ resistor.
-We then build the (presumably a multistage amp) circuit
-It drives a load with a resistance of 100Ω
-Output voltage across the 100Ω resistor must be 7V (pp) sin-wave

The only BJT's we use are 2N2222 (β=100), and the MOSFET can be any (resonable) simulated model (I used it with Vth about 1.5V)

I have built Figure 2 (FET model), but I have a pretty pathetic excuse for a sin wave. How can I restore my sin wave?

Figure 3 is as close as we can come with the BJT model. It looks like all we can do is level shift it and double the input. It oscillates between 3.1 and 4.3...

Any suggestions on either of these predicaments? Thanks!:confused:


I'm a little concerned about your MOS topology. Is it supposed to be just an output stage push-pull or something? I don't see a need for that, unless i'm missing something in your directions.

I like the topology of your BJT circuit... try using the same for the MOS.

Also... in your BJT circuit, it looks like your load resistor is sitting on your emitter... you don't want that i don't think. Put a decoupling cap off the emitter and take the load resistor from there to ground.

Another note... i think 5uf and 10uf coupling caps are going to GREATLY reduce your LF response (i.e. your gain at 80hz will suffer).

Ron H

Joined Apr 14, 2005
Are you also stuck with the 10V supply?
Does the 100Ω load have to be AC coupled to the output, or can output stage bias current flow through it?
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Thread Starter


Joined Apr 22, 2009
I am allowed to do whatever I want, as long as I use what is given in that first figure (lame I know). The professor that usually teaches electronics is on sabbatical this year, so we had an adjunct, and thus, learned the reader's digest version of the reader's digest version of transistors. He is rarely prepared for lab, and just kind of makes things up on the fly...

I am trying the suggestions here, and will let you know...thanks for the help so far