EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: abdulbadii on July 04, 2022, 05:41:07 am
-
Anyone outright understand then sincerely help explain the function of C4 in Q4 E (C6 on Q5 E) ?
(https://i.imgur.com/uEj1J7D.jpg)
-
This is a bypass capacitor that is often used in a common emitter amplifier. It drastically increases voltage gain and lowers input impedance of the amplifier.
There are a lot of articles describing how the common emitter amplifier works and formulas to calculate the values for given amplifier parameters. Just search for "bypass capacitor in a common emitter amplifier" and you will find a lot of articles.
Series resistor is less common, but it can be used to limit the gain.
-
C4 and C5 lowers the impedance in emitter thus increasing the transistor's stage gain with frequency (the higher the frequency the lower impedance of the capacitor C).
Without the capacitor the stage gain will be reduced and given by the ratio of collector vs. emitter resistor values.
The higher the frequency the lower the emitter impedance with the capacitor (and the higher the stage gain).
In your case the impedance in emitter will go from 330ohm (stage gain aprox 4) at low frequencies (around DC) to 30ohm (stage gain aprox 40, 330ohm parallel with 33ohm) with high frequencies.
Tip: learn the LTSpice simulations, LTSpice is free and handy..
-
Think of those caps as short-circuits for AC that are simultaneously open circuits for DC. This lets you bias the transistor (with DC) where you want it to be, while letting it get optimal AC gain.
-
You can find the original presentation and discussion of this design (from 2012) here:
https://eevblog.com/forum/projects/5-transistor-esr-meter-design/