Fet Input

Fet Input

I need to built a discrete op amp in a package doesn't exceed 3" on any side. Please help????

Here are the specs that I need to meet:
Design an amplifier which has the following capabilities:

1. Two inputs one plus , one negative

2. Capable of driving a 50 K ohm load to + 10 V

3. Frequency response DC - 10MHz + 2 db.

4. The input impedance must be at least 1 Megohm

5. The output impedance must be less than 50 ohms

6. Only discrete devices (BJT, FET, TUBES) may be used.

7. This amplifier will be packaged in such a way that it does not exceed more than 3 inches on any side.

8. Temperature range: 25 -- 100oC

9. The gain must be between 1000 - 1500

That gain is quite low for an opamp, most are well into the millions.

You have no offset voltage to worry about, no supply current spec, no negative voltages to worry about.

The two problem areas are the 10MHz response, presumably open loop. That will be difficult to achieve. If that is a response to 10Mhz before the gain falls off rather than a unity gain bandwidth specification, it will be VERY difficult, as that implies a unity gain bandwidth of 1000*10M = 10GHz!!!

And the temperature is the second problem, operation at 100C is difficult.

To keep the frequency response I'd stay with FETs, and keep the bias point up in the 10-20 ma range.

Look around on the internet for designs, keeping those points in mind.

Here is a discussion forum I found on that exact design. And I do mean exact!

http://www.physicsforums.com/showthread.php?t=90081

And the responses seemed to say that the design is impossible. State of the art is about 1GHz for a monolythic design. With discrete components, you will be lucky to get 1/10 th of that.