Push-pull output stage, 1M$ question - Page 1
Forum section: electronics / projects

author: wolf32d
author_rank: Contributor
timestamp: March 18, 2016, 03:07:31 pm
content: I'm fiddling with a couple of TIP 142 /147 complementary darlington BJTs. I'm trying to build a "power" audio output stage (BTW the TIP transistors are a bit overkill for my project).I'm using an op amp (OP07) to provide unity gain in a closed-loop configuration (fig 1) and at the same time I'm using a couple of diodes to provide anti-crossover biasing (like in fig 2). My question is: is it good practice to use the bias diodes even if the opamp guarantees no distortion & good linearity? Or are they just superfluous?

author: wolf32d
author_rank: Contributor
timestamp: March 18, 2016, 03:32:48 pm
content: Quote from: blueskull on March 18, 2016, 03:25:30 pmThanks! If I understand correctly, in DC applications, like in a power supply output stage, they aren't needed.

author: Zero999
author_rank: Super Contributor
timestamp: March 18, 2016, 03:36:44 pm
content: There will be some crossover distortion, even with the diodes.Pass transistors are a better way of boosting the power capability of an op-amp, with minimal crossover distortion.R1 & R2 should be chosen so Q1 & Q2 just about turn on, due to the op-amps quiescent current.

author: wolf32d
author_rank: Contributor
timestamp: March 18, 2016, 03:45:09 pm
content: Interesting topology! I've never seen that kind of circuit! Finding those 33 mOhm could be a little bit of a PITA.

author: Zero999
author_rank: Super Contributor
timestamp: March 18, 2016, 03:58:32 pm
content: Quote from: wolf32d on March 18, 2016, 03:45:09 pmThe original schematic was for 10A of output current. If you just want a low power amplifier to power a speaker than 1R will do.I've also seen it done with no emitter resistors but they're a good idea to prevent thermal runaway.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 19, 2016, 12:11:48 am
content: The op-amp only "guarantees" that performance under asymptotic time.This is more obvious if you use a more accurate op-amp model: integrator rather than gain stage. (Indeed, the traditional suggestion of "infinite gain", without any consideration of bandwidth, can only possibly guarantee one thing: oscillation!)An integrator, placed in a feedback loop, will act to reduce its input error towards zero. It takes infinite time to reach zero. For all finite times, the error will be nonzero. The error will be significant (comparable to the signal level) where the integrator gain is near 1 (i.e., it has little capacity to correct errors at such frequencies).An LM358 has GBW ~ 3MHz, so has gain ~ 10 at 300kHz, and so on. Which means an error of 1/10th or 10% is possible at that frequency. At 20kHz, a THD around 1% is quite possible, and typical. The most offensive part about LM358 is that its class C output stage produces a chunky transition, which is sensible (i.e., the error signal is a short pulse of ~constant energy) at arbitrarily low frequencies; even at DC, for control loop applications, it can be irritating.... I'll accept my $1M payment via bank transfer now, thanksTim

author: Audioguru
author_rank: Super Contributor
timestamp: March 19, 2016, 03:23:38 am
content: You showed a circuit using ordinary transistors and diodes to reduce crossover distortion to be very low. You are using darlington transistors that need 4 diodes but you used only 2 so the crossover distortion is very high. The gain of any opamp drops above 10Hz to avoid oscillation when negative feedback is applied so the high frequency crossover distortion is not reduced enough.A transistor can replace the diodes and allow the idle current in the output transistors to be adjusted for minimum distortion. If this transistor is mounted on the heatsink of the output transistors then it can compensate for thermal changes in the output transistors the same as diodes.

author: Circlotron
author_rank: Super Contributor
timestamp: March 19, 2016, 12:01:19 pm
content: On your original circuit, try putting a 100nf cap between the opamp output and the joined emitters. This will enable the opamp to contribute to the output during the crossover period and also prevent it from running open loop during this time. Try a few different values.

author: opty
author_rank: Regular Contributor
timestamp: March 19, 2016, 12:38:30 pm
content: Quote from: wolf32d on March 18, 2016, 03:45:09 pmInteresting indeed. I was asking about similar configuration a while ago:https://www.eevblog.com/forum/beginners/non-standard-op-amp-configuration/

author: dom0
author_rank: Super Contributor
timestamp: March 19, 2016, 03:02:39 pm
content: It was covered here a few times, conclusively in that thread and in another (I think the title was about "headphone amplifier coupling capacitors" or something like that, totally unrelated). Anyhow, it's an interesting circuit, but not so much for audio power amplifiers.

author: dom0
author_rank: Super Contributor
timestamp: March 20, 2016, 02:33:49 pm
content: "R3 provides current limit and allows the amplifier to drive the load when the output is between 0.7 V and -0.7 V"It never fails to amaze me how many slides, appnotes and datasheets, even some textbook, propagate just wrong ideas. Crossover distortion in transistor amps never had anything to do with output voltage, it's all about the strong changes of transconductance of the output transistors when the load current changes sign. Output voltage makes no appearance.

author: Zero999
author_rank: Super Contributor
timestamp: March 20, 2016, 05:29:46 pm
content: Yes, that circuit will have crossover distortion. Bypassing the base-emitter junction with a resistor may help to reduce it a little bit but it'll still be significant. The resistor also robs the transistors of some base drive which will reduce the output power capability somewhat.

author: dannyf
author_rank: Super Contributor
timestamp: March 20, 2016, 10:23:36 pm
content: Quotethey are there for thermal protection and to lower the slew rate demand on the opamp.Unless you are talking about some low frequencies, OP07+TIP14x isn't a good combination here.

author: Zero999
author_rank: Super Contributor
timestamp: March 20, 2016, 11:34:42 pm
content: Two additional emitter followers will help to remove a lot of the crossover distortion.Replacing R1 & R2 with active current loads would help improve the performance.

author: dannyf
author_rank: Super Contributor
timestamp: March 21, 2016, 12:52:52 am
content: QuoteIt is a class a OPS.

author: Zero999
author_rank: Super Contributor
timestamp: March 21, 2016, 12:36:13 pm
content: Quote from: dannyf on March 21, 2016, 12:52:52 amNo it isn't.

author: calexanian
author_rank: Super Contributor
timestamp: March 22, 2016, 06:42:27 pm
content: Any particular voltage range and OP AMP in mind? When using these sorts of circuits there are manny little niggles in them and I have a few favorite recipes for them. What sort of load? Speaker or electronic type load?