Why is the gain of my amplifier so low? - Page 1
Forum section: electronics / projects

author: gcewing
author_rank: Regular Contributor
timestamp: July 12, 2021, 02:30:04 pm
content: I thought I understood common-emitter amplifiers, but apparently not...According to my calculations, this circuit should have a small-signal voltage gain of about 80.Calculated values:Vb = 5 * (10/32) = 1.56VVe = Vb - 0.6 = 0.963VIe = Ve / Re = 0.936mAVc = Ie * Rc = 2.06VMeasured values:Vb = 1.53VVe = 0.91VVc = 3.02VRc = 2.16kIe = 0.923mAGain calculation:gm = Ie / 25mV = 36.9 mA/VA = gm * Rc = 79.7Measured gain at 5-10kHz:vin = 30mVpp, vout = 450mVpp, A = 15vin = 50mVpp, vout = 1.12Vpp, A = 22vin = 100mVpp, vout = 2.82Vpp, A = 28Different specimens of transistor give slightly different gains, but nothing anywhere near the theoretical value.I realise there are many reasons a real circuit can deviate from theoretical behaviour, but I wouldn't have expected it to be so far out at such low frequencies.Anyone have any idea what's going on? Have I got the theory wrong? Is there something wrong with my circuit?

author: Zero999
author_rank: Super Contributor
timestamp: July 12, 2021, 02:37:12 pm
content: How is the input and output connected?What load is it driving?The input impedance is a bit less than the values of the biasing resistors in parallel and the output impedance equal to Rc.

author: gcewing
author_rank: Regular Contributor
timestamp: July 12, 2021, 02:56:54 pm
content: I'm measuring the input voltage right at the base of the transistor, so even if the input impedance is loading the signal source down a bit, it shouldn't make a difference to the calculated gain, should it?.The only load it's driving is my scope.Signal voltages are measured by scope using x10 probes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 12, 2021, 03:07:07 pm
content: Vc calculation error -- subtract it from +5. That will agree well with your measurement.Also how are you coupling the signals, and what frequency? And the scope is measuring the same thing both sides (p-p or whatever)?Note that scope RMS includes DC, you need AC coupling for that to read just the signal part.Tim

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 12, 2021, 03:29:12 pm
content: Unless you AC couple the input, anything you connect, like a signal generator, is going to override the bias and all bets are off.

author: TimFox
author_rank: Super Contributor
timestamp: July 12, 2021, 03:29:28 pm
content: The emitter capacitor (10 uF in your drawing) needs to be compared with 1/gm= 27, which gives a corner frequency of roughly 500 Hz, but that should be OK for your 5 - 10 kHz measurements.

author: magic
author_rank: Super Contributor
timestamp: July 12, 2021, 08:20:59 pm
content: 30mV Vbe difference means 3x variation in collector current, that's not really a "small signal" anymore.

author: TimFox
author_rank: Super Contributor
timestamp: July 12, 2021, 08:29:00 pm
content: "Small signal" here, without emitter degeneration, usually means "small compared to the 25 mV in yourgmcalculation."To see how a BJT goes from small to not-so-small to large-signal behavior with sine wave input, seehttps://vdocuments.mx/communication-circuits-analysis-and-design-clarke-hess.htmlK K Clarke and D T Hess "Communications circuits: analysis and design", Addison-Wesley 1971, pp 104-113.Trigger warning: Bessel functions.

author: gcewing
author_rank: Regular Contributor
timestamp: July 13, 2021, 07:10:37 am
content: Quote from: TimFox on July 12, 2021, 08:29:00 pmPoint taken, although I'm not sure that's the whole story. If that were the only reason, I'd expect the gain to get closer to the calculated value as the signal gets smaller, but the opposite seems to be happening.I'll try some smaller signals. It's a challenge to accurately measure such small signals with my scope, but I'll see what I can do.

author: magic
author_rank: Super Contributor
timestamp: July 13, 2021, 07:35:51 am
content: Make sure there is no 10x error due to attenuation being set wrong on the probes or in the scope.Try to scope base and emitter at the same time, maybe something is wrong with the capacitor.Isn't the collector signal quite distorted?

author: mikerj
author_rank: Super Contributor
timestamp: July 13, 2021, 09:30:43 am
content: Collector/Emitter swapped?

author: Zero999
author_rank: Super Contributor
timestamp: July 13, 2021, 09:46:05 am
content: Quote from: mikerj on July 13, 2021, 09:30:43 amThe voltage measurments don't indicate that. Reversing the collector and emitter, would drastically reduce the hFE. This would increase the base current, pull VBdown, reduce IEand increase VC.

author: JimRemington
author_rank: Regular Contributor
timestamp: July 16, 2021, 12:43:33 am
content: LTSpice simulation predicts voltage gain of 75 at 10 kHz. 20 mVp-p in, 1.5 Vp-p out.

author: xavier60
author_rank: Super Contributor
timestamp: July 16, 2021, 02:29:12 am
content: Quote from: gcewing on July 13, 2021, 07:10:37 amMeasure the signal voltage at the Emitter also.

author: Zero999
author_rank: Super Contributor
timestamp: July 16, 2021, 07:33:21 am
content: Quote from: JimRemington on July 16, 2021, 12:43:33 amHere's the asc file I made a few days ago, but forgot to post.