Small signal BJT amplifier - high gain

349

07:03:43

This circuit provides a fairly high voltage gain of  40 (32dB) using two common emitter stages, the main design challenge of this circuit is making sure one stage does not excessively load the previous stage or source, the input 10k resistor simulates a source of moderate impedance, a Darlington pair configuration provides a low impedance output ideal for driving a power amplifier, this circuit does not use every trick in the book base bootstrapping and global feedback can improve input impedance and gain.
As a simple rule you should avoid single stage gains above 10 to prevent nonlinearity.

published 11 years ago

Sine_eyed

11 years ago

I've only recently begun to study BJT's as amplifiers, so when I ask, it's because I don't know. So far it seems to me that proper biasing is THE critical element when designing a one or multiple stage circuit. I found the math involved on the internet and I can comprehend it fairly well I think- I was just wondering if you or anyone had further insight, tips or tricks used when deciding how a particular transistor should be biased..

justinmcg67

11 years ago

Is the key here chaining multiple Class A amplifiers together? I am not an expert on amps, i really have no idea...

Sine_eyed

11 years ago

Yeah- configurations like this are not uncommon. I have a couple already modeled in my private file

Sine_eyed

11 years ago

..as well as completed real life builds. However building from a schematic is one thing, conception and design is another. My original query stands..

faceblast

11 years ago

breadboard, and lots of trimpots

faceblast

11 years ago

just saturate everything. bias is for valves. the future is digital

hurz

11 years ago

My 2 cents, we have seen many here on EC and this is all in all ok, not to much gain, good frequency behaviour, input impedance can even be lowered to 1k without distortion but probably better noise figur. What i dont like is the darlington output. After you have fined two stages amplification you endup with just a primitive emitter follower. I am sure this darlington wasnt in the original circuit. Again, all in all not bad.

Chryseus

11 years ago

@Sine-eyed Generally you want to bias to maximize signal swing and minimize the output impedance, since input and output impedance are related you often have to compromise between them, usually you want the input impedance to be 10x the previous stage or source impedance, as a result most bjt amplifiers are optimized for a specific application, Art of Electronics is a good source of information.

Chryseus

11 years ago

@hurz There are certainly a lot of better amplifiers, usually these days you'd just use an op-amp, however for high frequency applications a discrete amplifier is usually preferred, although the common-emitter isn't a good choice due to miller capacitance, the Darlington pair was intended although I agree not the best method.

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