BJT Preamp Topologies

301

02:32:53

These 3 basic topologies are based upon the classic designs known as (from left to right) common emitter common base and common collector but with a few twists. The uncommon thing about the first circuit is that the bias design enables the transistor to enter a stable temp state and work in a safe bias zone. The advantages are: Greater reliability than other designs. Higher temperature stability than other designs.The twist on the second circuit is the emitter decoupling capacitor. Usually in the most simple case the signal source is not decoupled in any way and the DC passes freely through it. Simply a resistor in series limits the current to some point but DC through the signal source is NEVER good. And now the most bizzare topology of all. Remember how in books the common collector circuits are said to always have A voltage gain less than one Ku<1? Usually this is true but by simply eliminating the negative feedback you get voltage gain as well, that almost becomes independent from the transistor hfe.

published 10 years ago

joefranklin

10 years ago

Awesome. I've been working on something similar.

thebugger

10 years ago

I recommend using the third circruit (depending on your needs) because its simpler and it seems to have a nice low distortion fearures. The voltage gain is a bit low ( maybe a few times) but as I said depending on the purpose it's really helpful

Chryseus

10 years ago

The first circuit isn't that stable since as the transistor beta varies the bias point will shift, the emitter degeneration however does help a lot making it usable in most applications, the second circuit is only really for specific applications such as RF due to the very low input impedance of the emitter, the last circuit is rather interesting design however the practical use is limited in my opinion as it offers no real advantage over a common-emitter.

Voltransistor

10 years ago

Cool. I been working on an FM radio and it uses a pre-amp similar to the middle one, instead of the capacitor, it uses an inductor.

thebugger

10 years ago

Yeah the common base will do for high frequency amplification. Try it and let me know.

Voltransistor

10 years ago

@thebugger-Thankyou! :-) FM preamp are very sensitive to responding to a perfect station. A little movement will change the station. It when great, tfa, It took a while.

thebugger

10 years ago

Tell me about it. It can be disrupted just by someone's presence close to it. Nice to hear it went well.

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