basic transistor amplifier - Page 1
Forum section: electronics / projects

author: ykurban
author_rank: Contributor
timestamp: April 07, 2020, 02:04:56 pm
content: How do you calculate gain of this simple amp?

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 07, 2020, 02:09:14 pm
content: Are you familiar with a bipolar transistor's h-parameters?Since most resistors are well bypassed with capacitors, at a mid-frequency the circuit is reduced to a grounded emitter with a single collector resistor load.Although you can calculate the gain of the stage by itself, for the actual gain you require to know the load impedance and the source impedance.

author: graybeard
author_rank: Frequent Contributor
timestamp: April 09, 2020, 07:27:42 am
content: If you can assume beta is large then the base voltage is:VB = R8 / (R8 + R9 + R10 + R11) * 9V = 2.96VThe emitter voltage is:VE = VB - VBE = 2.96 - 0.68 = 2.28VThe emitter current is therefore:IE = VE / R12 = 84µAsince we assumed beta is large:IC = IEthe transistor transconductane is:gm = IC / VT = 3.36 mSwhere VT = k * TK / q = 25mVthe mid band voltage gain from the base to the collector is:AV = gm * R4 = 3.36ms * 2.2KOhms = 6.72As schmitt trigger said the load and source impedances will alter this result.

author: duak
author_rank: Super Contributor
timestamp: April 09, 2020, 04:44:14 pm
content: Graybeard, I remember doing those calculations in college and was able to determine the DC conditions. I remember the next steps used IE and the 25 (or 26 mV) constant but not that it gave the transconductance. Use it or lose it, indeed.

author: graybeard
author_rank: Frequent Contributor
timestamp: April 09, 2020, 07:29:26 pm
content: Quote from: duak on April 09, 2020, 04:44:14 pmIn my case it has been more than 30 years since I taught this as graduate assistant, but I still use it for work.

author: bson
author_rank: Unknown
timestamp: April 09, 2020, 07:30:01 pm
content: Quote from: graybeard on April 09, 2020, 07:27:42 amShouldn't this be gm * (R4 + R5 + R6)?Edit: oh wait, nvm, I wasn't awake enough to note it's an AC property. R4 alone leads to ground.

author: xavier60
author_rank: Super Contributor
timestamp: April 09, 2020, 10:02:28 pm
content: AV = 67.2 seems high. At college we were taught Re = 29/I, I in milliamps. We weren't told were the 29 came from.Re = 29/0.084mA = 345ΩSo I get an AV of 6.37.

author: graybeard
author_rank: Frequent Contributor
timestamp: April 09, 2020, 11:50:27 pm
content: Quote from: xavier60 on April 09, 2020, 10:02:28 pmI slipped a decimal place in the quick calculations. gm=3.36mS not 33.6mS

author: vk6zgo
author_rank: Super Contributor
timestamp: April 10, 2020, 02:33:18 am
content: I hate the complex looking bias circuit, although it is mainly just an artifact of the simulator rendition.My first approximation for gain, just looking at the 2.2k load, was "Gain--- not very much!"Luckilygreybeard&xavier60have "done the hard yards" so I didn't have to !I remembered from back in the mists of time, "gm Rc" (actually, it was "gm Rp" when I first met it).

author: graybeard
author_rank: Frequent Contributor
timestamp: April 10, 2020, 07:38:52 pm
content: Quote from: vk6zgo on April 10, 2020, 02:33:18 amOnly for pentodes and perhaps tetrodes.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: April 11, 2020, 02:28:27 am
content: It has also been a long while since I did this but my calc agrees with greybeard. 6.7 looks right.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 11, 2020, 03:29:18 am
content: Quote from: graybeard on April 10, 2020, 07:38:52 pmIndeed, but transistors look more like pentodes than triodes.