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ASK AN ENGINEER

Dear Engineer,

Last week, I was working in the lab, trying to model a small signal response across an NMOS transistor amplifier. The experiment wasn’t working, and neither was my laptop. I had borrowed my girlfriend’s laptop, so when I opened it to look up the lab manual, I saw a Facebook conversation with her sister. I guess she thinks there’s not enough intimacy in the relationship, and I feel really bad.

Anyway, our anniversary is in 10 days, and I really want to make it up to her. But I’m really at a loss for how to breathe new, romantic life into our relationship. For her sake and for mine, because I really love her and would hate to lose her. Please help me, Engineer!

Signed,

Hopeless Romantic


Dear Hopeless Romantic,

It sounds like a number of things could be going on, but thankfully all of them are easy fixes. First of all, you need to make sure your Function Generator is set to high impedance (Hi-Z) mode. Remember, we are dealing with signals on the order of millivolts and milliamps! Make sure your DC biasing voltage is a reasonable value: 15 V would suffice.

Now, there are many different configurations, but one of the simplest is a Common Source amplifier. This is analogous to a Common Emitter amplifier with Bipolar Junction Transistors, but if you’ve never worked with BJTs worry not; MOSFETs can be understood and used on their own.

The theory behind a biased small signal amplifier is the separation of the steady-state DC large signals and the time-variant AC small signals. Make sure your capacitors are properly polarized in order to filter out the DC bias from the amplification circuit itself – you just the DC sources to keep the MOSFET in saturation region!

Assuming a source impedance of 50 Ω is a safe bet. The resistor values should be specified in your lab manual. Make sure your source terminal is grounded through a resistor, not connected to negative DC! With simple amplifier circuits, often trial and error is the best method to correct problems.

Signed,

Engineer