Help with second stage of a dual stage common emitter amplifier

Thread Starter

Whiskey-Kiss

Joined Dec 22, 2017
7
Hi guys,

For education purposes I'm attempting to chain together 2 CE amplifiers with an emitter follower between, but as you can see from the screenshot below, the waveform is obviously undesirable! The output from the first one was good, with a voltage gain of about 300, but this second stage is all wacky, with emitter DC voltage higher than the base voltage.

I added the emitter follower because, as I understand it, the Zin of the following stage should be about 10x the Zout of the previous stage, and 2 CE amps with a coupling cap wasn't giving me that ratio.

What's going on here? How can I chain these together, with the aim to increase gain, successfully?

EDIT: Couldn't seem to embed imgur image, so schematic is in attachment!
EDIT II: There is no load hidden behind the osc window, its just the osc leads going to the Q2's collector

 

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AlbertHall

Joined Jun 4, 2014
11,506
Let's assume both CE stages have a gain of 300. The overall gain would then be 90,000. With a 1mV input that would give an output of 90V which, with a 16V supply is clearly impossible. Turn down the input. Try it with 50uV.
 

Thread Starter

Whiskey-Kiss

Joined Dec 22, 2017
7
Thanks guys!

The problem was, as you said, that the gain of the stages together was far too high. I was so focused on achieving high gain I didn't stop to think there might be too much!

Rather than reducing the input, which would have worked too, I removed bypass cap C2 as Danko suggested, tweaked the DC bias, and it's functioning perfectly now.
Gain is around 2400, as you calculated, which is great.

Bordodynov, though I didn't use your schematic, that is an elegant looking amplifier which I will certainly simulate and study.

Thanks again for your time and for solving my problem, and Merry Christmas!
 
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