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Old 12th March 2004, 01:24 PM   #1
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Default simple amplifier with mosfets

Hi all, will this little baby work in real life ? -
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Old 12th March 2004, 01:38 PM   #2
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I may be wrong but ... no.
It looks like C3 is providing an a.c. short across the upper transistor.
Lose C3.
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Old 12th March 2004, 02:43 PM   #3
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Also, it will be very difficult to achieve the correct biasing. One solution would be to use a Vbe (or Vgs) multiplier, mounted to the heatsink to allow thermal tracking.

Good luck!
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Old 12th March 2004, 03:28 PM   #4
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The biasing difficulty is this.
With no input signal you, ideally, want the source voltage to be half the power rail, or 43V. The voltage across R2 will be about 0.6V. So the voltage across R3 will be 42.4V and the current through R3 will be 42.4/860k = 49uA. This current must be the sum of the current through R2 and the base current of Q3. R2's current is about 0.6V/56k = 11uA, so Q3 base current must be 38uA. So Q3's base current is dominating the current through R3. Now, Q3's collector current needs to be about 43/10k = 4.3mA. Knowing Q3's base and collector currents we can estimate it's beta to be 4.3m/38u = 113.

So the source voltage will sit at half the supply voltage only if the beta of Q3 is 113. The trouble is that the beta characteristic varies from one device to the next, often 50% or more, and within a single device it varies with Ic and temperature and Vce. So the odds of beta being 113 are very slim. If the beta was 50% out so would be your source voltage. You need a bias arrangement that is relatively insensitive to beta.
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Old 12th March 2004, 10:00 PM   #5
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Ditto on the above. Also:

I would consider splitting R1 into two values and attaching the
+ terminal of the 100 uF cap between them for bootstrapping,
which is what might have originally been intended.
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Old 13th March 2004, 02:27 PM   #6
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Default Well how about this ?

Thanks alot for your comments. I made some changes regarding the bias, and the bootstrap.

It's an easy little amp - will it run ?
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Old 13th March 2004, 03:09 PM   #7
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The three diodes aren't enough to bias the mosfets. You will need 4-6 volts (compared to 2.1 V) and some temperature compensation.
/Per-Anders (my first name) or P-A as my friends call me
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Old 13th March 2004, 03:32 PM   #8
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Per Anders is right. Use a Vbe multiplier as I suggested above, and mount it on the heatsinks. To address the bias stability of the input transistor as Traderbam noted, add an emitter resistor on the input transistor.
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Old 13th March 2004, 03:52 PM   #9
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alternatively, you can use a pot in place of the three diodes to get this thing going. It wouldn't be able provide any thermal stability, which is critical in your design as there are no source resistors on Q3/Q4.

the voltage drop on R4/R7 is about 40v (86/2-3v for the Vgs drop on Q3). and you need about 6v drop on between the gates of Q3/Q4 so you need a pot about 6/40*(3300+3300) = 990ohm so a 1.5k pot should do.

I would strongly suggest a pair of source resistors.
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Old 13th March 2004, 04:45 PM   #10
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Have a look at SuperFlea:

Just look at one half. Of course the ultimate in simplicity for a complementary design is the ZV5
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