Directly coupled high gain BJT amplifier - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: January 07, 2022, 02:08:00 am
content: after making a mic preamp with BJT's, I got obsessed with trying to get a really flat response out of it,my demands were:50-15khz flat response, don't care much for phase shiftmust have a phase inverter, so one output must be 180 degrees out of phasemust have at least 300x gain, with it adjustablemust have input impeadance of over 50k.no coupling capacitors, preferably even directly coupled from the electret mic (outputs about 2V DC)must run from 9 to 40V fairly wellmust use 4 transistors or lessmust all be self biased, so any variation will transistor would still make a usable ampas you can see my demands are quite insanebut maybe you have some suggestions on how to go about it,I have tried bootstrapping, driving the signal into the emitter, none of those really satisfied my demands, either I had crappy frequency response or it didnt have much gain.

author: Ian.M
author_rank: Super Contributor
timestamp: January 07, 2022, 04:28:31 am
content: You are asking for something rather similar to a discrete OPAMP.IMHO the most difficult requirements to meet are likely to be:Quote from: ELS122 on January 07, 2022, 02:08:00 amIt typically takes five or more transistors to make a decent discrete OPAMP, and a wide supply voltage range makes biassing far more complex than you can get away with if the supply voltage is known, and well regulated. Additionally, you want complimentary outputs, which will certainly increase the transistor count.

author: magic
author_rank: Super Contributor
timestamp: January 07, 2022, 07:54:51 am
content: Possibly something like Nelson Pass supersymmetry. Shown below with BJTs from the similar OPA1632 integrated circuit datasheet.You would remove that "Vcm" thing and its NPNs, replace them with two resistors.Output impedance is high (determined by the resistors above) without additional buffering.The PNPs could be made common emitter instead of common base for more gain but this thing will have low-ish input impedance when modified to work with feedback.Without feedback, gain could be controlled by degenerating the differential pair with variable resistor(s), not sure if it would do 300(?).Swap PNP and NPN if your discrete PNPs have less noise than NPN (complementary pairs often do).

author: Marco
author_rank: Super Contributor
timestamp: January 07, 2022, 10:00:56 am
content: A common emitter amplifier with a sziklai pair for amplification, an emitter follower buffer, leaves one transistor to cascode the input transistor for linearization.

author: Marco
author_rank: Super Contributor
timestamp: January 07, 2022, 05:48:09 pm
content: So let's say that is driving a DC coupled XLR input, it sees a 18V common mode. Let's say it's AC coupled instead, now the output transistors of your amplifier have no bias current. Am I seeing this wrong?Your input impedance is nowhere near 50k.What kind of electret microphone has an internal fet connected as a source follower?

author: Marco
author_rank: Super Contributor
timestamp: January 08, 2022, 12:53:43 am
content: I didn't notice you were taking phantom power from the output.