What is the PSRR of a one-transistor amplifier? - Page 1
Forum section: electronics / beginners

author: cvanc
author_rank: Frequent Contributor
timestamp: March 24, 2019, 10:59:33 am
content: This question arises out of the troubleshooting thread linked below. I honestly don't know what the PSRR of a simple common emitter stage is expected to be. I spent some time looking for a citation and so far have come up empty. What does the group mind here have to say about this? Can you cite a reference?Schematic for the circuit that triggered this inquiry is attached, but I'm looking for the general rule on this issue. Thanks!https://www.eevblog.com/forum/repair/can-you-double-check-my-troubleshooting-here-(old-kenwood-receiver)/

author: exe
author_rank: Unknown
timestamp: March 24, 2019, 11:13:46 am
content: Had similar questions as well for years. This article answered them:http://sound.whsites.net/articles/followers.html.Basically, you can use a simulator to verify the design without building it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 24, 2019, 11:21:39 am
content: About zero.The collector output impedance is high, so the voltage at the output is essentially Vcc - Ic * Rc. The Vcc isn't reduced at all, it has gain 1, so the part of the output due to variation in Vcc is a ratio of 1, or PSRR of fuck all dB.Perhaps a better way to think of it is a shift in the ground reference for the signal. Suppose you follow an NPN common emitter stage with a PNP common emitter stage. The PNP's input is relative to its emitter, which is now near Vcc. The Vcc term in the NPN's output cancels out with this, and PSRR is quite good (roughly Rc/Rearly).Early effect is the name given to collector output impedance (i.e., change in collector current for a change in voltage -- the incremental resistance). It depends on transistor type and bias current. Typical figure would be in the 100s kohms for a preamp circuit like this, so PSRR in the 20-40dB range would be expected.Note this assumes signal frequencies. That is, where the base bias divider doesn't contribute because the coupling caps and source impedance dominate. Base bias being derived from Vcc (as in the traditional four resistor, one transistor, 2-3 capacitor, common emitter circuit) does have an effect at other conditions, of course.You can always improve Early effect say by stacking transistors in cascode. In that case, the PSRR about doubles, again contingent on being able to ignore bias dividers of course.Tim

author: cvanc
author_rank: Frequent Contributor
timestamp: March 24, 2019, 11:43:25 am
content: Quote from: T3sl4co1l on March 24, 2019, 11:21:39 amYeah, that's more or less what I was guessing. Many thanks for the explanation.In the specific troubleshooting case I have before me I nonetheless suspect the hum isnotcaused by rail ripple. See linked message - I'm barking up the right tree here, right?Thanks again.https://www.eevblog.com/forum/repair/can-you-double-check-my-troubleshooting-here-(old-kenwood-receiver)/msg2292795/#msg2292795

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 24, 2019, 01:57:06 pm
content: Right, PSRR is a design issue, not a repair issue. If it was a bad design, it would always be bad, no matter how many components or boards you replace.For repair, you need to find which component(s) have failed, introducing that noise. Possibilities include saturated amp stages, which consequently run at low PSRR. Or if supply ripple itself is so large, it's causing stages to saturate sometimes, introducing noise that way.In short: replace all electrolytics and try again. You can maybe measure some (ESR meter) and see if they're okay, but the best strategy is to dump them all and then see if there are other issues present.Tim

author: cvanc
author_rank: Frequent Contributor
timestamp: March 24, 2019, 04:00:21 pm
content: Quote from: T3sl4co1l on March 24, 2019, 01:57:06 pmI already did, with no change in the symptom.The 1uF input & output coupling caps are new, the 47uF rail bypass is new, I've added 470uF across the 47 just to see if it helped (it did not). Ripple on the rail bypass cap is 3.7mV which seems reasonable.The key thing, to me, is this: After re-capping this stage, removing any of the 4 input coupling caps stops the hum in that channel. This suggests to me the hum is not coming from this stage at all, but from somewhere upstream.Here's where it gets weird: The next thing upstream is the volume pot, and the volume pot does not affect the level of hum at all (even though it works normally for signals passing thru).It's a head-scratcher.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 24, 2019, 08:34:30 pm
content: Ah, recapped, good.What happens if you lift the input side of the coupling cap, and tie it to ground?Freeze spray?Tim

author: cvanc
author_rank: Frequent Contributor
timestamp: March 24, 2019, 09:36:02 pm
content: Quote from: T3sl4co1l on March 24, 2019, 08:34:30 pmUhhh... I'm gonna need to put it back together before I can get you an answerGive me a day or so. Meanwhile, I've got some iffy soldering & blackened connectors to clean up

author: MrAl
author_rank: Super Contributor
timestamp: March 25, 2019, 07:13:52 am
content: Quote from: cvanc on March 24, 2019, 10:59:33 amHi,Any voltage input can be treated as an input signal source. This means the PS rail can be treated as a signal input too.The main input to the amp normally has a relatively low level and gets amplified but a lot, but it is low level and we assume that it can change the output by a reasonable amount. A typical input could be 10mv.The PS rail however may not contain a low level AC component signal with ever regular filtering. Tt could easily be 1v or 5v depending on how well they made the PS. The PS ripple is the main thing here, and will contain a fundamental frequency component of 120Hz or 100Hz (and some lower level harmonics) depending where you live. This sounds like a hum in the output speaker.So the test then is to apply an AC signal of 100Hz or 120Hz to the PS in addition to the DC signal, and measure the output AC. You can then take the ratio:r=VACps/VACoutand that will give you the ratio of input to output.This will vary from amplifier to amplfier but if you post the schematic of your particular amplifier we can estimate the ratio.