Re: HV gain stage with fewest number of transistors

From: Winfield Hill (hill_a_at_t_rowland-dotties-harvard-dot.s-edu)
Date: 11/10/04 

Date: 10 Nov 2004 12:07:33 -0800

Jim Thompson wrote...
>
> Winfield Hill wrote:
>
>> Given that 1200V MOSFETs are readily available, it's easy to
>> make up to +/-600V dc-coupled amplifiers, using a totem-pole
>> output stage like we show in AoE, fig. 3.75, but with an added
>> negative rail and level-shift stage. Here's the basic idea:
>>
>>. basic high-voltage MOSFET dc amplifier
>>. by Winfield Hill
>>.
>>. + supply rail -----------------+-------,
>>. | |
>>. R11 |
>>. | D Q4
>>. +--+--G
>>. C2 | | S
>>. ,--R4--||-, | C |
>>. IN | | | B--+ Q5
>>. o--R3--+--|+\ | | E |
>>. | >---+--R5--, X | R10
>>. ,--|-/ | | | | out
>>. | E | '----+-+--R12--o
>>. | gnd -+- B Q1 | D1 |
>>. | | C +--|<|----+
>>. | | | +---R13---+
>>. | | | | |
>>. +---------- | -- | ------- | -+--R2--+
>>. | | | | | |
>>. R1 R13 | C3 | '--||--+
>>. | | E ,-||--+ C1 |
>>. gnd +- B | | |
>>. | Q2 C R8 | R9
>>. | | | D Q3 |
>>. R14 +---++--G C4
>>. | | | S |
>>. | R6 C | gnd
>>. | | B--+
>>. | | E | Add a zener across
>>. | | | R7 each FET gate!
>>. | | | |
>>. - supply rail --+----+----+----'
>>
>> The circuit is inexpensive and works well at low frequencies but
>> it can be further enhanced for a -3dB bandwidth over 100kHz.
>>
>> As you can see the circuit includes short-circuit protection. My
>> finished designs generally have more parts to add features such as
>> HV-output monitoring, reduced cross-over distortion (at point X),
>> increased slew rate, improved power-supply rejection, etc.
>
> Looks like a MOSFET twist on my MC1554 design, for which I won a
> coffee bet with Tom Frederiksen... he didn't think I could do an
> all_NPN design.

 It's often called a totem-pole output stage, and I first ran into
 it in the late 60s in DTL logic gates. When did you do the mc1554?

> Problem: Loop gain is higher when upper half of output stage is
> active than with lower half.

 Yes, and the pulldown gain depends on the load. This all forces
 an overly conservative compensation.

> I bet I can guess method at "X" ;-)
>
> ...Jim Thompson

 Yep, there are several good candidates working towards class AB.
 A big problem is one doesn't know much about each MOSFET's linear
 gate operating voltage... For one series of these HV amplifiers
 I even designed a little simple floating servo system. 

-- 
 Thanks,
    - Win

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