Will both these circuits behave the same for peak detection? - Page 1
Forum section: electronics / beginners

author: Infraviolet
author_rank: Super Contributor
timestamp: October 11, 2023, 07:09:31 pm
content: I've been using circuit A as a peak detector with a bleeding resistor (R38/R40) to discharge the storage cap (C18/C20) over time and an extra (R39/R41, C19, C21) low pass filter stage to eliminate any ripple, but am wondering if there's any reason NOT to switch to design B.In A a base-emitter junction of an NPN transistor acts as a diode for a peak detector, because this BE drop is often more precisely tuned at manufacture and more predictably temperature variable than the drop of an actual diode. The collector is connected to the base simply to prevent it floating and maybe doing something weird.B would have the advantage of a higher input impedance, whereas design A is such that at the very peak of an incoming waveform C18 presents almost zero resistance so a relatively large current very briefly flows. If the source supplying the signal had a series resistance within it then B would lose less voltage and have the capacitor charged up right the way to ( V_peak - V_B_E_drop ) , with no source resistance losses affecting V_peak, and as far as I can understand would otherwise be exactly the same in behaviour. Any noise or change in voltage on the power rail would seem to have no effect, as the power rail would just be supplying current for the emitter to output at a voltage of ( V_peak - V_B_E_drop ).Can anyone suggest circumstances where B would be inferior, or problems it can have that A is immune to?Thank you

author: coromonadalix
author_rank: Super Contributor
timestamp: October 11, 2023, 09:00:30 pm
content: https://electronics.stackexchange.com/questions/112347/how-to-make-a-peak-detector-circuithttp://www.learningaboutelectronics.com/Articles/Precision-peak-detector-circuit.phpA is acting as a rectifierB with proper biaising will act more as a switch since when polarized the transitor will push the collector voltage to the emitterthis circuit should be betterhttps://freecircuitdiagram.com/344-single-transistor-half-wave-peak-detector-circuit/

author: Infraviolet
author_rank: Super Contributor
timestamp: October 11, 2023, 10:08:05 pm
content: Have I misunderstood something? For an NPN in this configuration the emitter cannot ever go above the base voltage minus the BE drop? Regardless of what happens at the collector?When you say "acting as a switch" you mean one which tops up the emitter voltage from the collector until such time as the emitter would get above V_base-V_be_drop, at which point v_emitter rises no further?

author: Benta
author_rank: Super Contributor
timestamp: October 11, 2023, 10:32:44 pm
content: It all depends on your input signal.If it can supply enough current to charge the cap, fine. Use a diode (or transistor connected as one).If it can't, use the transistor.

author: Infraviolet
author_rank: Super Contributor
timestamp: October 12, 2023, 02:00:28 am
content: If the signal source can supply enough current for cap charging without significant voltage loss, is there any harm in using the version B collector-to-power-rail method?

author: Kim Christensen
author_rank: Super Contributor
timestamp: October 12, 2023, 02:17:29 am
content: You could put a resistor in series with the collector to avoid drawing too much peak current from the power rail.That's where this circuit is a bit better.You can change R1 to more precisely control the attack time of the peak detector:

author: wasedadoc
author_rank: Super Contributor
timestamp: October 12, 2023, 01:45:00 pm
content: Quote from: coromonadalix on October 11, 2023, 09:00:30 pmNo, that is incorrect.

author: David Hess
author_rank: Super Contributor
timestamp: October 12, 2023, 07:35:44 pm
content: Quote from: Kim Christensen on October 12, 2023, 02:17:29 amTektronix used all kinds of variations of that, but always with transistors, sometimes single transistors, sometimes Darlington pairs, and sometimes Sziklai pairs.Jim Williams published a similar design, but with a diode connected transistor on the input to compensate for the Vbe drop, and a transistor on the output to charge the capacitor, as shown below.