# Help with second stage of a dual stage common emitter amplifier

#### 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.

#### Danko

Joined Nov 22, 2017
1,135
What's going on here? How can I chain these together, with the aim to increase gain, successfully?
Take away capacitor C2.

#### AlbertHall

Joined Jun 4, 2014
11,506
Take away capacitor C2.
That will reduce the gain of the second stage to just 8 which I expect is not the desired result.

#### Danko

Joined Nov 22, 2017
1,135
That will reduce the gain of the second stage to just 8 which I expect is not the desired result.
300 x 8 = 2400. Not so bad. With capacitor it will working with distortion. May be decrease value of emitter resistor to 300 Ohm then gain 300 x 27 = 8000.

#### Bordodynov

Joined May 20, 2015
2,907
300 x 8 = 2400. Not so bad. With capacitor it will working with distortion. May be decrease value of emitter resistor to 300 Ohm then gain 300 x 27 = 8000.
When replacing the resistor 300Ohm, it is necessary to select resistors in the voltage divider (reduce the lower resistor)

#### 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!

#### AlbertHall

Joined Jun 4, 2014
11,506
I removed bypass cap C2 as Danko suggested, tweaked the DC bias, and it's functioning perfectly now.
Remember that you added the emitter follower to match impedances. Now that the second CE stage has an un-bypassed emitter resistor it has a high input impedance so the emitter follower is no longer required.