SillyAmp

One could ac couple, unless it's a DC amplifier, then back to two parts is better. The pnp is an mpsa92.

+275 IN | --47k--, RL | | |/e +---- OUT ,---| | | |\\ | gnd | |-' +---|| 600V | |-, MOSFET 100k | | Rs __|______|_____ -275

Winfield Hill a écrit :

That one has a highly optimized 2nd order distortion. Should sound good and satisfy the most exigent tube aficionados :-)

--
Thanks,
Fred.

How about a feedback resistor from "OUT" to PNP(e) ??

But there's probably not enough loop gain to reduce the distortion by much.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave

Jim Thompson a écrit :

Before doing that, you'll need to properly bias the input stage. Exercise left to the student :-)

--
Thanks,
Fred.

Are there any "students" here? They certainly don't step forward and attempt to learn :-(

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave

Mini-nits: both my optocouplers were the same, npn's, and my input-side supplies were +-2, to keep idle current down:

Is not! You're just jealous that you don't have 400 volt optocouplers in any of your ic libraries.

John

Actually, it's not so bad with a significant Rs. That's because the power high-voltage MOSFET will be operating in its sub-threshold region, where its gm is nearly as high as a BJT at the same current. With the high supply voltage and a modest ratio RL/Rs =3D 50, for 500V out with 10V in, Rs > 1/gm will keep the distortion under control.

Actually, as mentioned above, with Rs =3D 0, the loop gain could be sizeable, and rightly, done it could work reasonable well. The pullup capabillty is rather wimpy and there are several DC offset sources.

+325 ZERO | -- Rx --, RL =3D 150k IN | | 5W -- 20k --+-- 1M --+ -/+5V | | |/e +---- OUT ,---| | +/- 250V | |\\ | G =3D -50 gnd | |--' +----|| 600V | |--, MOSFET 2.2M | __|________|_____ -275

A real circuit would be more complicated, like some I've posted before, including an opamp, a totem-pole pullup transistor, current limiting, and compensation.

I was just referring to the absence of bias for the PNP, assuming the input signal was symmetric :-)

I'd probably reduce the 2.2M to 22k or such, which is about 200uA and

60mW for the PNP, for a better gate positive slew rate and GBW product.

--
Thanks,
Fred.

Yep, but be careful, that 200uA would have to come from the PNP summing junction, making a high output offset to correct. Maybe 2.2M is too high, but consider, Crss is only 3pF for an FQP1n60 above Vds =3D 25 volts, and with Vgs about 3 volts at Id =3D 2mA, we get a rising slew rate of S =3D I/C =3D Vgs/RC =3D 0.5V/us. 220k would give us 5V/us. If this stage is placed inside an opamp feedback loop, then we can begin to get aggressive about the currents. Unless a pullup transistor is added, external capacitive loads will seriously slow down this wannabe beast anyway.

+300 ZERO | QUICK-n-DIRTY -- Rx --, RL =3D 150k HV AMPLIFIER IN | | 5W -- 20k --+-+-- 1M --+ -/+5V | | |/e +---- OUT ,---| | +/- 250V | |\ MPSA92 | G =3D -50 gnd | |--' +------|| 600V MOSFET | |--, FQP1N60 220k | __|__________|_____ -260

I'd feel better if there was a gate zener, and maybe a resistor in series with the PNP collector; I hate all-silicon paths from -260 to ground. Then go ahead and add a source resistor, to get some current limiting and improve linearity.

If you want silly, how about an optocoupler/mosfet cascode? Or better, ie sillier, an opto+depletion fet cascode!

Actually, that's getting interesting.

John

John Larkin a écrit :

I thought about that last one as a replacement to the HV optos when you first 'published' your idea here a couple of years ago. But now that's 4 active parts. Well six, including the leds.

--
Thanks,
Fred.

The zener is mandatory. Diode protection for the PNP, yep. Safety current limiting is wise, although after making hundreds of such amps with mpsa92 trannies, I had only one failure and it blew the mpsa92 wide open. Lots of energy stored in the supply caps after all.

+300 ZERO | QUICK-n-DIRTY -- Rx --, RL =3D 150k HV AMPLIFIER -/+5V IN | | 5W -- 20k -----+--+-- 1M --+ | | ,--|>|--+ | | | | | |/e +-- 1k --- OUT +-----| | +/- 250V | |\ MPSA92 | G =3D -50 gnd | | 47k | | |--' +----+--|| 600V MOSFET | \|_ |--, FQP1N60 220k /_\ | | | 12V | __|____|______|_____ -260

v+ | | r | +-----------out | | d in---------+ g ---+ | s | | | | a c | k ---> b | | opto e | | | | gnd +-----+----- v-

3 parts total! And you can get low-voltage optos as duals, so you can make it full totem-pole, class B, 500 volts p-p out, with three parts!

Apex, cry your heart out.

John

v+ | | r | +-----------out | | Supertex d depletion-mode in--/\/\---, g ---, | s | | | | a c | k ---> b | | opto e | | | | gnd +-----+----- v-

Awesome. Don't forget a resistor for the opto.

Another silly KYAG circuit ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave

?? Really? KYAG? How so? ?? That looks like a relatively-well behaved circuit, with excessive gain. But it does cry out for a feedback loop.

Concern for opto NPN punch thru during power-up... I could go for a zener or some-such in there.

I've been musing really retro... maybe a mixed toob/silicon amplifier stage ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave

Silly man. No HV power supply I'm aware of has a fast enough risetime at power-up to come close to creating such an issue. Maybe if one was to use a HV switch in the power supply line, but then the blame would lie in using the switch (been there, done that, oops), not with the circuit. There is a danger if (when) the output is instantaneously shorted, ameliorated with a small output R.

____ V+ hv --o o-----, +200 | 150k, 2W | +/-170V +---- 2.2k -------out | | Supertex DN2540 d depletion-mode g ----, s | in--/\/\---, | 10V | | +--| b | k | e | | | | ____ V- gnd ---+-------+---o o---- -180

A higher-voltage version (+/-300V output) can be made with a Supertex dn2470. That's a 700V depletion-mode MOSFET, the record holder for such parts. Mouser stocks 'em.

This circuit would benefit from feedback. A Darlington NPN would minimize the feedback-node bias current, and an offset resistor could provide for bipolar inputs and correct for the LED and BJT drops at the summing junction. A resistor, say Vgs/I = 470 ohms, in the MOSFET's path would limit the pulldown current to a few mA, to avoid overheating the FET.

V+ hv --/\/\---/\/\---, John's HV amp idea +360 150k, 2W (2) | | Rf/Rin = 60 +/-300V ,- Rf--+ to 1mA Roffset | +---- 3.3k ----------- OUT +5 ----/\/\-----+ | Rin | | Supertex DN2470 IN ----/\/\------+ d depletion-mode -5 to +5 | g ----, | s | | | 10V | +5 --/\/-- c | +--|

On Mar 23, 4:37 am, "Winfield Hill" wrote: [....]

I'm going to suggest a modification or two:

Modified version

Adding the diode (or diodes) marked "NEW" partially compensates for the EB drop of the bipolar transistor.

If instead of a diode, we make that the E-B drop of a PNP emitter follower, we can use a non darlington for the NPN feeding the LED.

If we place a resistor in parallel with the LED we can run the NPN feeding it at a more well known current. This would make matching it with the PNP better.

The capacitance of the 10V zener slows things down.

Many K +V---/\/\----- ! 10V ------->!-----+----!