#957 Gain and Noise Figure in transistors

Episode 957
I look as some circuits from the ICOM radio. This is the microphone preamp

[bobdoritique]

One of your best videos, I think. THANKS!

[springwoodcottage]

Super interesting. Thank you for sharing!

[ats]

The noise factor formula isn't quite right. The noise in later stages are reduced by the product of the gain of all the previous stages, so the last term would be (F3-1) / (G1 * G2)

[pbaemedan]

The 4700 ohm resistor between the collector of the PNP and the emitter of the NPN is a feedback resistor.

[ovalwingnut]

Just as my popcorn was ready.... the (beyond interesting) show was over :/ I'm setting the play back speed to 50% next time :) Very COoL. It was like I was your neighbor and you would let me watch (that didn't come out right) as long as I kept my yap shut. This is all going on my resume, thanks so much. Cheers from So.CA, 3rd House on the right.

[keithfisk]

2:18 "a kind of upside down emitter follower" - I don't think so. If it was an emitter follower it would need an emitter resistor and the signal path would be via the emitter. The second transistor emitter is firmly grounded for signal frequencies by the 220 uf capacitor.

[pbaemedan]

Great introduction to noise figure, thank you. I would recommend you do some practical demonstration by injection of a controlled amount noise and measure it with both your scope and spectrum analyzer.

[paolomonai]

Good evening. As already underlined by other viewers, there is a mistake in the reported cascaded noise figure formula (Frijs formula) but the concept is clearly illustrated...regards from Italy.

[argcargv]

The three transistor amplifier has an interesting feedback including two germanium diodes in series. It looks like it might be performing some sort of AGC function. What does the 3k trimmer do to the microphone gain?

[Pixelwaster]

The cascade section bugged me a bit. Seen it somewhere but not in a radio or audio circuit. After a bit it hit me like a ten ton heavy thing - a throttle amplifier from a vintage train set.

Where does the dotted X off the 100k go to? I am tempted to simulate this in Spice to understand the base bias / negative feedback / collector limiter a little better. Never seen something like that.

Changing the gain on the first Q for FM (emitter bias via the 1SS53) is a one way to do it. I am guessing FM because the BA401 is an FM IF amp and the trace goes to the supply line. Not sure about the 2nd Q because I can't see where the wiper & 22k hook to.