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Old 20th September 2011, 03:44 PM   #1
Bigun is offline Bigun  Canada
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Default TGM5 - all-BJT Simple Symmetric Amplifier

This is a thread to capture my build of an all-bipolar transistor version of the Lazy Cat simple symmetric amplifier (SSA). The thread that inspired this build is here: Simple Symetrical Amplifier

There are a lot of designs and some great ideas on that thread, but I wanted somewhere to capture this specific build and my experiences - I hope they are helpful to others.

I'm planning to build 3 channels; this will be the main amp for my HT system front and centre speakers. I believe this design has the right qualities for the application - low distortion and good damping factor.

The schematic is attached.

Question: there are capacitors C11 and C12 on the output driver stage to prevent parasitic oscillations of the associated complimentary feedback pairs (Q8-Q10 and Q9-Q11). Should these capacitors be connected to the bases of the driver devices before or after the base-stopper resistors ??

EDIT (4 March 2012) SOME USEFUL POSTS LATER IN THIS THREAD

Details of Hagerman Vbe multiplier are in post 22: TGM5 - all-BJT Simple Symmetric Amplifier

The final as-built schematic shown in post 87: TGM5 - all-BJT Simple Symmetric Amplifier

First Sound! post 179: TGM5 - all-BJT Simple Symmetric Amplifier

Photos of Completed modules post 186: TGM5 - all-BJT Simple Symmetric Amplifier

LTSpice file attached in post 226 (any conflict between this and the schematic in post 87, take the as-built one from post 87): TGM5 - all-BJT Simple Symmetric Amplifier

BOM for as-built version shown in post 242: TGM5 - all-BJT Simple Symmetric Amplifier
Attached Images
File Type: jpg TGM5v3SCH.jpg (190.1 KB, 3698 views)
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Last edited by Bigun; 5th March 2012 at 01:49 AM.
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Old 20th September 2011, 04:36 PM   #2
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Quote:
Originally Posted by Bigun View Post
...Should these capacitors be connected to the bases of the driver devices before or after the base-stopper resistors ??
Hi
I do not know
but did you try TMC like in prevoius versions of TGMs?
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Old 20th September 2011, 05:41 PM   #3
Bigun is offline Bigun  Canada
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I wasn't planning to use TMC on this SSA amplifier. I don't think it needs it. I feel the primary benefit of TMC is extending the effectiveness of the -ve feedback to higher frequencies for amplifiers weighed down by large Cdom capacitors. I'm hoping that the SSA amplifier will require very little compensation and so no TMC. Of course it probably depends on keeping a tight pcb layout with not so much stray inductances and capacitances.
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Old 20th September 2011, 09:59 PM   #4
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Default I dont understand ............

Current flows across R13 then Q3 ( base of Q3 is positive ) then Q2 ( base of Q2 is at zero ) then across R6 ................... then I cant understand ?????? cannot work in class A must be class B using R12 for an input circuit ????????

I am out of buisness ..........................
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Old 21st September 2011, 01:33 AM   #5
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by audiofan View Post
Current flows across R13 then Q3 ( base of Q3 is positive ) then Q2 ( base of Q2 is at zero ) then across R6 ................... then I cant understand ?????? cannot work in class A must be class B using R12 for an input circuit ????????

I am out of buisness ..........................
I've drawn my schematic the same way as lazycat, but if you were to go and look at the JLH class A amplifier you'd find it easier to understand - what Lazycat has done is make it symmetrical. Let me try and clarify it a bit.

Q2 is an input device. The current flow through it depends on the voltage across the base-emitter junction. The voltage at the base is the input signal (wrt ground). The voltage at the emitter is a feedback signal from the output (wrt ground). In other words, Q2 is the 'error amplifier' that is both the input device and -ve feedback device. It's a single device instead of the classic Long Tail Pair.

To see how this feedback signal is derived look at how the output is connected back through R12 and then to ground through R4. Together, R12 and R4 form a potential divider, the classic arrangement for -ve feedback. The junction of R12 and R4 is the -ve feedback signal and it's fed to the emitter of Q2 via R6. This R6 provides some additional degeneration (more about this later). So Q2 is a single input device just like the JLH class A amplifier. The output from Q2 is at it's collector. The collector is cascoded by Q3 - strictly this additional device Q3 isn't necessary but it provides some benefits.

So the output from Q2 collector is a varying current flow, via Q3 it flows through R13, which generates a voltage across the base-emitter junction of Q5. Q5 dutifully amplifies the signal and feeds it to the output stage which buffers it, thus providing lots of output current.

This is a symmetrical amplifier and all that I have described above has a mirror image which provides a complementary signal. So, Q1 is a mirror of Q2; they both see the same input signal and the same -ve feedback signal. However, being complementary (n-type and p-type) they have opposite polarity outputs which are later recombined by the voltage amplification stage. Q4 is the cascode for Q1 and is a mirror of Q3. The output from Q1, via Q4, produces a voltage across the base-emitter junction of Q6, which is a mirror of Q5. And Q5 and Q6 form the voltage amplification stage that recombines the two opposite polarity input signals from the input stage to produce an amplified voltage (wrt ground) for the output. The output is biassed into Class AB by the voltage drop across Q7.

Back to R6, and it's partner R5. They add series resistance to the emitters of their respective devices Q2 and Q1. These additional resistances 'swamp' the lower intrinsic emitter resistance of the input devices which vary between devices because of the nature of manufacturing tolerances. This helps ensure that the two input devices, Q1 and Q2 behave more closely in the same way, as they are designed to do here to minimize distortion. Same story for the degeneration resistors R17 and R18 on the voltage amplification stage of Q5 and Q6 - they help with matching the behaviour of the two devices which we want to operate in concert with each other.

I hope this helps?
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Last edited by Bigun; 21st September 2011 at 01:42 AM.
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Old 21st September 2011, 02:41 AM   #6
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Default Now Isee

Now I understand the circuit and I see the relation with JLH.

At zero input signal the input circuit is bias by R7 & R8 to keep alive input transistors ...........

Merci beaucoup

Have a good day
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Old 21st September 2011, 07:08 AM   #7
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Quote:
Originally Posted by audiofan View Post
Current flows across R13 then Q3 ( base of Q3 is positive ) then Q2 ( base of Q2 is at zero ) then across R6 ................... then I cant understand ?????? cannot work in class A must be class B using R12 for an input circuit ????????

I am out of buisness ..........................
hehehe at first glance SSA front-end looks a mess but when one understand the principle it is child's play simple.

Audiofan, here is a real full balanced version of SSA.

Bigun thanks for your SSA explanation. I will follow TGM5 thread and be supportive if asked.
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Old 21st September 2011, 10:24 AM   #8
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WOW, A CFP triple output stage for you, Bigun? This is a change (and a stability worry).
'Not sure that the output device is the right place to sense temperature for bias compensation, though. Since Self's work, one of the drivers would seem to be the more appropriate and simpler sensing point.
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Old 21st September 2011, 02:02 PM   #9
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by Ian Finch View Post
WOW, A CFP triple output stage for you, Bigun? This is a change (and a stability worry).
'Not sure that the output device is the right place to sense temperature for bias compensation, though. Since Self's work, one of the drivers would seem to be the more appropriate and simpler sensing point.
I think for BJTs the triple output has a lot going for it even though it requires the two caps for stability I don't think these caps need be large enough to impact sonics in any way.

I used this output with my TGM3, also with current feedback. It sounds good but I didn't like the dc offset drift and had to install a dc-servo (single transistor dc-servo mind). The symmetrical front end from lazy cat is a nice way to solve this problem I hope - in a way, my TGM5 is a symmetrical TGM3.

designing TGM3 - output Triples

It's true to say that in a CFP the 'slave' device is under the grip of the driver (or pre-driver in TGM5) so that temperature control of the slave device is now a different prospect. BUT, in TGM5 it is only the driver that is in CFP configuration, the output pair are in the standard EF configuration and so they need temperature compensation.

One thing I am going to do differently is run the drivers at lower current. In my TGM3 I ran the drivers in high current, like Roender. But in TGM4 I don't want to dissipate so much standing current and although technically not as good, I believe it will be more than adequate for the task at hand. The standing current through the drivers will still be in-line with 'conventional' practice and yet still have the benefits of a triple in terms of isolating the VAS from the non-linear Class AB switching output stage.


Quote:
Originally Posted by Lazy Cat View Post
I will follow TGM5 thread and be supportive if asked.
thank you !
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Old 21st September 2011, 02:36 PM   #10
Bigun is offline Bigun  Canada
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What kind of open-loop-gain plot are we hoping for for this amplifier ?

With 15pF of Cdom I get a simulated response as attached:
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File Type: jpg TGM5v3olg.jpg (124.3 KB, 3215 views)
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