Armstrong oscillator question - Page 1
Forum section: electronics / beginners

author: Quantumplate
author_rank: Regular Contributor
timestamp: July 01, 2025, 04:30:34 pm
content: Working my way thru a 70s book by Leo Rozman. On this page i'm a little confused about the paragraph below the diagram. If anyone can flesh out his explanation a bit that would be great. Also specifically where it says "long discharge time thru R2' did he mean R1 ?I guess i don't really get what the time constant of C2-R2 have to do with the operation of the circuit either. As i see it the bias is set by R1 & R2 and the tank circuit 's AC swings that up and down via coupling cap C2 turning the transistor off & on.Thanks!

author: TimFox
author_rank: Super Contributor
timestamp: July 01, 2025, 04:45:31 pm
content: R1 was added to R2 to provide a bit of startup bias, and should be much larger than R2, so R2 dominates the time constant with C2.That time constant, if too long compared with the oscillation frequency, can produce “squegging”, a nasty parasitic low-frequency amplitude modulation of the oscillator.This is a topic in its own right, so I suggest you look for references to squegging (a real word, not a joke) in the literature.

author: Quantumplate
author_rank: Regular Contributor
timestamp: July 01, 2025, 06:29:33 pm
content: Quote from: TimFox on July 01, 2025, 04:45:31 pmThanks Tim, i love this new word. I wish i could say my wobbulator's tickler is squegging, but technically that's never gonna be correct...Back to my brain ache: i don't get how C2 can discharge thru R2 - the r.h.s. is at a lower voltage than the bottom end of R2. Assume i don't know much in your answer...Thanks again

author: TimFox
author_rank: Super Contributor
timestamp: July 01, 2025, 07:49:46 pm
content: OK: to make a practical (harmonic) oscillator, there are two unrelated mandatory functions:1. Frequency-dependent network to give zero total phase from output to input at only one frequency (the obvious resonant circuit in your drawing).2. Amplitude stabilization: the basic amplifier in the oscillator must have more gain than required for sustained oscillation (gain x feedback > 1), and the amplitude network normally reduces the gain below that to get stable amplitude.In traditional L-C oscillators, starting with the vacuum-tube circuits, there is a diode (B-E in your case, G-K for tube, G-S for JFET) that conducts on one extreme of the sine wave in the "forward" direction (I normally think in terms of NPNs or tubes, so think hard about the PNP variant you posted), and the other extreme is therefore in the reverse direction, where the effect on the device gain is to reduce the gain.In these circuits, the effective gain of the transistor itself is reduced as the amplitude increases.As the amplitude increases from start, the circuit reaches an equilibrium (unless it squeggs), where the large-signal gain of the device is reduced to the point that gives (gain x feedback) = 1.Note the term "large-signal gain": at these AC voltage levels, the transistor is driven out of its linear voltage gain region, so the important gain (seen at the base) is at the oscillating frequency, where the sinusoidal signal is due to the resonant circuit, even though the collector current waveform is seriously distorted.

author: Quantumplate
author_rank: Regular Contributor
timestamp: July 01, 2025, 08:51:01 pm
content: That's far more eloquent an overview than i could give, but i follow and sort of knew that already. But to come back to me getting bogged down in the details - did the author mean to say R2 or R1 when talking about C2 discharging ?? (his line in brackets)

author: TimFox
author_rank: Super Contributor
timestamp: July 01, 2025, 09:38:17 pm
content: The load on C2 is the parallel combination of R2 and R1, where R2 is smaller and dominates the combination.

author: fourfathom
author_rank: Super Contributor
timestamp: July 02, 2025, 03:12:28 am
content: It helps me to understand the circuit if I change the transistor to NPN and reverse the battery polarity (as God intended.) And add a ground symbol. Yes this is entirely unnecessary, but it removes one level of confusion for me.

author: iMo
author_rank: Super Contributor
timestamp: July 02, 2025, 07:12:10 am
content: LTspice is your friend..

author: Quantumplate
author_rank: Regular Contributor
timestamp: July 02, 2025, 12:56:32 pm
content: Thankyou all. I think my problem was trying to combine ac and dc analysis at the same time and tying my brain in knots.