Question regarding a simple amplifier circuit - Page 1
Forum section: electronics / beginners

author: sentry
author_rank: Contributor
timestamp: August 23, 2018, 05:47:26 pm
content: Okay, so I understand that REis used to stabilize the transresistance instability in the transistor to combat distortion by lowering the amplification gain for the DC signal. CEis used to bypass REfor the AC signal and thereby obtaining the full gain of the amplification, but only for the AC signal. What I don't understand is why this construction doesn't distort the AC signal because its unaffected by the negative feedback caused by RE? I would expect the AC signal to also be sensitive to the small hFEfluctuation caused by temperatur and current changes?

author: David Hess
author_rank: Super Contributor
timestamp: August 23, 2018, 07:15:01 pm
content: It does distort the AC signal to the extent that the transistor's own transconductance changes with current which changes with output voltage level.This configuration is used to provide maximum AC gain while maintaining DC stability which would otherwise allow the transistor to suffer thermal runaway.

author: Zero999
author_rank: Super Contributor
timestamp: August 23, 2018, 07:38:56 pm
content: The hFEmakes no difference to the transistor circuit's voltage gain, only its current gain. It's easy to build a transistor circuit to have a far higher voltage gain, than current gain.REis required because it would be virtually impossible to bias the transistor so roughly half the supply voltage appears across RCin its quiescent state, without it. VBEis too variable and the transistor has too higher transconductance to allow that to happen.See link below for more information:http://www.ee.ic.ac.uk/pcheung/teaching/aero2_signals&systems/transistor%20circuit%20notes.pdfThe distortion can be avoided, at the cost of reduced gain by removing CEor adding a resistor in series with it.

author: sentry
author_rank: Contributor
timestamp: August 26, 2018, 04:36:53 pm
content: Okay, so I constructed this simple amplifier and I was expecting to see the output signal phase shifted 180ofrom the input signal, but I only see 80o. Is that because of the output capacitor? I'm using a BC548 NPN transistor in my circuit.

author: Zero999
author_rank: Super Contributor
timestamp: August 26, 2018, 05:55:37 pm
content: Quote from: sentry on August 26, 2018, 04:36:53 pmIt might be.What frequency did you test it with and what's the load impedance?

author: sentry
author_rank: Contributor
timestamp: August 26, 2018, 06:20:19 pm
content: Quote from: Hero999 on August 26, 2018, 05:55:37 pmI tested it with frequencies between 3 - 5 MHz. No load connected to the output, only my scope probe. Is that the reason, because I dont have a load connected?

author: Audioguru
author_rank: Super Contributor
timestamp: August 26, 2018, 10:39:31 pm
content: At the radio frequencies you are using, the electrolytic capacitors have inductance that causes phase shift and frequency response errors. Their values are way too high for radio frequencies, much lower value ceramic capacitors should be used.Stray wiring capacitance also causes phase shift and frequency response errors at your radio frequencies.If you are using a solderless breadboard then its wiring and rows of contacts have a lot of inductance and stray capacitance.

author: Zero999
author_rank: Super Contributor
timestamp: August 26, 2018, 10:48:48 pm
content: Quote from: sentry on August 26, 2018, 06:20:19 pmThat frequency range will be above the upper cut-off frequency of the amplifier, so the gain will start to fall by then and phase shift will be greater than 180o.Another thing is the 'scope probe will present a capacitive load at that frequency, which will also add more phase shift to the circuit.I suspect you have a phase shift of something like 440o.Try 3kHz to 5kHz.I think the biasing is also a little off. What's the collector voltage, when no signal is applied? It should be about half the supply voltage, give or take a volt or so.

author: sentry
author_rank: Contributor
timestamp: August 27, 2018, 07:48:54 pm
content: Quote from: Hero999 on August 26, 2018, 10:48:48 pmSo I tried to lower the frequency to 3 - 5kHz. The result is much better now, especially when the signal is 4kHz. The phase shift is almost spot on now (180o). Excellent:DThank you for your help and suggestion.