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 Post subject: Circuit question
PostPosted: Jul Tue 02, 2019 10:35 pm 
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What exactly is the following circuit called?

I have a feeling that I should know it, but the name isn't coming to me at the moment.

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 Post subject: Re: Circuit question
PostPosted: Jul Tue 02, 2019 10:48 pm 
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Darlington


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 Post subject: Re: Circuit question
PostPosted: Jul Tue 02, 2019 11:01 pm 
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It is even when the two bases are tied together and the two emitters are tied together?


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 Post subject: Re: Circuit question
PostPosted: Jul Wed 03, 2019 12:03 am 
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Tube Radio wrote:
What exactly is the following circuit called?

I have a feeling that I should know it, but the name isn't coming to me at the moment.


Yes, you should know it, it is a Push-Pull circuit.

This is a Darlington:


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 Post subject: Re: Circuit question
PostPosted: Jul Wed 03, 2019 12:29 am 
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Oh ok thanks.

It is complementary symmetry.


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 Post subject: Re: Circuit question
PostPosted: Jul Wed 03, 2019 3:42 am 
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Also called "totem-pole"---or is that reserved for 2 NPNs stacked?

I think push-pull would mean the same symmetry as a P-P tube circuit.

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 Post subject: Re: Circuit question
PostPosted: Jul Wed 03, 2019 4:06 am 
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I remember it as a complementary pair output. I built a couple of medium power amps of that type back in the 70s I believe from Popular Electronics magazines. In this amp it looks like a complementary pair of darlingtons.
http://www.swtpc.com/mholley/PopularEle ... 7_pg30.jpg

I'm pretty sure I have the Lil Tiger and Plastic Tiger amp boards in a cigar box somewhere. They worked good for the times.


Last edited by forumuser on Jul Wed 03, 2019 4:17 am, edited 2 times in total.

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 Post subject: Re: Circuit question
PostPosted: Jul Wed 03, 2019 4:14 am 
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Quote:
Also called "totem-pole"---or is that reserved for 2 NPNs stacked?


Can be either 2 NPN or a PNP/NPN totem. Mainly described that way when it is used to drive another (larger) transistor.

https://microcontrollerslab.com/totem-pole-tutorial/

Rich


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 Post subject: Re: Circuit question
PostPosted: Jul Wed 03, 2019 4:42 am 
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Thanks for the links. I'll read them tomorrow.


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 Post subject: Re: Circuit question
PostPosted: Jul Wed 03, 2019 5:44 am 
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Transistors Push-Pull:
https://www.google.com/search?client=firefox-b-1-d&biw=1293&bih=949&tbm=isch&sa=1&ei=bDIcXf9xxNv6BK-Sm5gH&q=transistors+pushpull+amplifier&oq=transistors+pushpull+amplifier&gs_l=img.3...3708.7246..7580...0.0..0.135.1049.3j7......0....1..gws-wiz-img.fi6MoFaPK0Q


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 Post subject: Re: Circuit question
PostPosted: Jul Thu 04, 2019 3:44 pm 
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THe best solution is to use a dynamic bias circuit. This will allow you to easily set the Idle current (no signal) and very significantly reduce crossover distortion. This circuit was quite popular in descrete power amplifiers of the 70s and 80s.

The OP's circuit is a particular type of push-pull amplifier, known as a complimentary symmetry current amplifier, so named because it uses a complimentary pair of PNP and an NPN devices, in an Emitter follower configuration. As shown at "A" the circuit will have considerable crossover distortion at low volume settings, thus, is not well suited to audio amplifier amplifiers.

To improve crossover distortion, a couple basic approaches can be used. One is to stack diodes in the base circuit to parallel the B-E junctions in the transistors as shown in "B". This works better, but there can still be cross over distortion and or Idle current problems, which can be reduced by inserting additional resistors as shown in"C". Because of differences in each transistor, resistors must be hand selected, not good for mass production.

Idle current and distortion can be well controlled by using dynamic biasing with an additional transistor, as shown in "D" and "E". To minimize thermal considerations the bias transistor can be mounted on the heat sink along with the output power devices.

In "E" resistors are added in series with the bias transistor, and capacitors coupled to the output. These resistors are added to allow a higher output with a given power supply voltage, by keeping the voltage across the bias circuit more or less constant as higher AC output voltages approach the power supply rails. This technique is know as "boot strapping" as in the old saying "pulling yourself up by your boot straps".

Image

A totem pole amplifier uses two devices of the same type, either PNP or NPN, one of which is driven by a phase inverter device. Totem pole output circuits were common in very early power transistor audio amplifiers. Totem pole output circuits were also later used in TTL logic ICs because of the low saturation voltage in the Logic "0" state.

Image

Here is a simple low power fairly low distortion audio amplifier using boot strapped dynamic biasing.

Image

As for emitter follower vs darlington. An emitter follower, is either a single device, as shown in "A". Two complimentary devices, that are so connected so as to provide current gain in either a positive or negative transition.

A "Darlington" is comprised of two transistors of the same type, either PNP or NPN, with the emitter of one, connected to the base of another as shown in "B". The current gain of a Darlington connected pair is approximately the product of the current gain's of the individual devices.

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 Post subject: Re: Circuit question
PostPosted: Jul Thu 04, 2019 3:52 pm 
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Excellent Mike, very good and complete description.

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 Post subject: Re: Circuit question
PostPosted: Jul Thu 04, 2019 4:13 pm 
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I am just throwing this in as a suggestion for a simple high gain amplifier in addition to the very nice presentation given earlier in this thread,

The open loop gain of the TL431 shunt regulator when used as an audio amplifier is about 50dB (550).

Its slew rate should be high enough to remove much of the crossover distortion in the output stage. Purists may want to add the Class A biasing diodes and gain may be controlled by adding a series resistor on the input which also increase the input impedance.


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 Post subject: Re: Circuit question
PostPosted: Jul Thu 04, 2019 4:30 pm 
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The transistors will be driven by a TL-494 switch mode power supply chip with two sets of transistors for push pull and will drive a pair of csd16570q5b power FETs.

Originally used 2N3904 and 3906 transistors but those initially in my testing didn't seem to have enough oomph to drive the high gate capacitance.

Now I have another question.

The circuit runs at 150KHz with the transistors and fets close together.

Is it possible to move the transistors off the board for testing and just run wires to the board or would that affect the circuit? As in can I use a breadboard to plug the transistors into without negatively affecting the circuit?

Also for driving power FETs will that transistor configuration be good as is or should it be modified?

Very good explanation, Mike.


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 Post subject: Circuit question
PostPosted: Aug Thu 22, 2019 3:55 am 
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Thanks Don... I considered a voltage divider like adding the appropriate resistor to ground, but decided to go with the zener instead. Not sure how adding a resistor to ground would help - and it may trigger the FET if too value low? BTW, is pulling your hair out called fun?
Woody, KZ4AK

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