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Old 10th June 2008, 10:08 PM   #1
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Default ccs-loaded sziklai

While designing an output stage i had to face the choice between EF or CFP, but when i finally dediced for the CFP one i had an idea:

Could the output stage driver collector resistor be replaced with a ccs? Why? This should give two nice features: Make the driver transistor run hotter (and have less largue signal distortion) and improove the turnoff speed.

Since i have never seen something like this in any design, is there any reason why this shouldn't work well (I assume it will need a bit of compensation).

With a mosfet output the gain developed by this arrangement would be incredible and if some brave one makes it stable it could provide super-low open-loop distortion.

Could someone give me... feedback?
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Old 10th June 2008, 10:30 PM   #2
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DC bias conditions will be not very well defined, moreover a typical ccs may need around a Volt to stay in active region, not really a problem with mosfets but hard with bipolar output devices.
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Old 10th June 2008, 11:42 PM   #3
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Quote:
Originally posted by darkfenriz
DC bias conditions will be not very well defined, moreover a typical ccs may need around a Volt to stay in active region, not really a problem with mosfets but hard with bipolar output devices.

I planned to get the voltage with a diode on the output emmiter or simply by powering the driver with slightly higher voltage (i usually power the small-signal stages from a cheap $15 transformer as this makes it much more stable).

I had always thought that the current depended on the collector resistance (the one from the power transistor R1-R33). I don't see this as an issue.
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Old 11th June 2008, 03:31 AM   #4
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ionomolo,
what FET are you going to use?
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Old 11th June 2008, 03:41 AM   #5
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It will work well, with very high OLG as you say and vanishingly low Zout. Use a 220R gate stopper, and choose around 5mA on the npn driver with a 9140 mosfet as output device.

Run it in Class A only.

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Old 11th June 2008, 04:48 AM   #6
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Not exactly CCS replacing collector resistor, but adding totem pole after collector resistor may improve turnon/turnoff of mosfet CFP
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Old 11th June 2008, 05:37 AM   #7
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The easy way with bipolars is to buffer the outputs and it will work well both ways..
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Old 11th June 2008, 09:54 AM   #8
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Hi Ionomolo
---simply by powering the driver with slightly higher voltage---

Without using a CCS but a simple resistor in a classical Sziklai configuration, powering the driver from a slightly higher voltage but clamped to the voltage of the power supply of the power transistor, should have the benefit to be able to reverse bias it, just like it is done in the standard four emitter followers push-pull by the capacitor in parallel with the resistor between the emitters of the drivers. It should give slightly lower HF distorsion.
http://www.diyaudio.com/forums/showt...92#post1536092
I think that Douglas Self had a short sentence about this possibility.
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Old 11th June 2008, 03:44 PM   #9
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Quote:
Originally posted by AKSA
It will work well, with very high OLG as you say and vanishingly low Zout. Use a 220R gate stopper, and choose around 5mA on the npn driver with a 9140 mosfet as output device.

Run it in Class A only.

Hugh
Thanks sincerely. I have a couple of IRFP9240 here that might work.

I have just noticed that in CFP configuration the Cgs is not bootstraped by the load as it happens on the source follower. With a capacitance arround 1.4 nF and a gate resistor of 200 Ohms this would have a -3dB corner at 560 KHz, which makes it comparable to the slowest bjt's. Since the swing of the bjt may be several times larger than the needed for full output at DC (assume 1.5V to get 6A at 4S transconductance) this may allow it to keep amplifying one decade further to 5MHz, which is enough bandwith as long as the phase shift does not make the whole thing crazy, but it does not allow much headroom for compensation.

I'm correct? Is there any workaround to increase output bandwith?
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Old 11th June 2008, 04:54 PM   #10
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Since the use of mosfets will give 4V of margin for the ccs, would it be imprescindible to cascode?

I think that a ccs made with 2 transistors may become inductive with frequency, and a RLC network inside a circuit with high gain makes a power oscillator for sure. Are the diode-biased ccs's better in that sense? would cascoding solve that problem? does the problem exist at all?
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