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Transistor cathode follower
Transistor cathode follower
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Old 10th September 2019, 04:09 PM   #11
GoatGuy is offline GoatGuy  United States
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Quote:
Originally Posted by PRR View Post
> Why not a simple high voltage NPN transistor such as a KSC5026M emitter follower

Because HIGH-voltage BJTs must have low current gain.

The KSC5026M barely tops hFE(min) of 10.
Well, as I said earlier, this is why the Darlington 2-transistor configuration is often deployed. For instance, the APT27ZTR 800 mW, 450 VCEO breakdown is 38˘ ea, and offers nominal hFE of 25 or so at 10 mA. Coupled to an APT13005SU, 60˘/ea … with heat sink able to handle 20 W, and with a 100 mA gain of 35, you get a mutual gain of 35 × 25 = 875 or higher.

Since the input impedance load is
ZIN = β ZLOAD where
β ≈ hFE ⊕ 1
for emitter-follower circuits, we can figure that a 25 kΩ load would deliver a 1 MΩ or higher input impedance.

Not bad for a couple-of-bucks solution.

Just saying,
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Last edited by GoatGuy; 10th September 2019 at 04:14 PM.
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Old 10th September 2019, 06:29 PM   #12
GoatGuy is offline GoatGuy  United States
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A diagram...
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File Type: png Screen Shot 2019-09-10 at 11.25.07 AM.png (36.4 KB, 79 views)
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Old 10th September 2019, 08:27 PM   #13
GoatGuy is offline GoatGuy  United States
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Update with ''more of the usual documentation''.

PS: this same diagram, substituting a lower voltage B+ power supply, can work quite well down to B+ = 20 volts or so. Of course, the dynamic range gets squeezed substantially. But on the upside, the maximum tension (voltage) of the capacitors also is similarly reduced. Making for less expensive, smaller and more compact caps. Also making for much lower voltage requirements for the power supply and for the Q1, Q2 transistors. Indeed, below about 150 V supply, easily obtainable Darlington-in-one-package transistors can be used.

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Last edited by GoatGuy; 10th September 2019 at 08:32 PM.
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Old 11th September 2019, 05:56 AM   #14
N101N is offline N101N
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The KSC5026M is a fake transistor aimed to be used as a switch. As an amplifier it will generate massive distortion.

Quote:
Capacitance is not an issue with Mosfets when used as source followers.
Crss does not increase much but it is always there. The series Darlington coupling involves a huge capacitance coefficient, that is, very high current gain and a very modest bandwidth.
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Old 11th September 2019, 07:49 AM   #15
6A3sUMMER is offline 6A3sUMMER  United States
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N101N,

One nice thing about a MOSFET is you can drive the gate voltage all the way up to the Drain Voltage.
So you can use it for large voltage swings as a Source Follower.

But take care choosing a MOSFET. The poor old IRF150 has the following: Crss 450pF at 10V Drain to Gate, Crss 50pF at 50V Drain to Gate.

Tube Cathode followers are normally not that bad (not the capacitance, and not the non-linearity of capacitance). Most MOSFETs are much better than the IRF150. But you get the point.
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Old 11th September 2019, 10:57 AM   #16
baudouin0 is offline baudouin0
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As the source follows the gate you don't see the capacitance:

The input capacitance of a source follower configuration shown in Figure 1 is defined as the sum of the gate to drain capacitance, and gate to source capacitance multiplied by, 1 minus the gate to source gain, so:

Cin = CGD + CGS(1 - AGS)

Where: AGS = RS / RS + (1/gm) if the drain resistance is much greater then the source resistance as is often the case (rd >> RS).


I was looking at STP3NK60ZFP which has internal zener, and has an insulated tab.

Agreed the valve is better but may require a elevated heater source. I think it 200v but 150v max cathode - heater is safer.
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Old 11th September 2019, 07:38 PM   #17
6A3sUMMER is offline 6A3sUMMER  United States
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baudouin0,

Yes, good formula. But those capacitance numbers change versus voltage. Look at the family of capacitance curves versus voltage. Plug in the numbers for one voltage, figure the total gate capacitance. Then plug in the numbers for another voltage, figure the total gate capacitance.

My main point was the changing capacitance. It can be a contributor to non-linearity.
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Old 12th September 2019, 05:47 AM   #18
baudouin0 is offline baudouin0
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The second formula should be

AGS = RS / (RS + (1/gm))
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