LTPI tweaking

[Alan0354]

A source follower can get quite a bit closer to the rails. I'm all for overdriving the power tubes. I'm just trying to avoid clipping the phase inverter in the process, since it's mosfet.

Make sure you use protection diode on the gate to prevent the gate from swinging over 15V either way. Read the spec, usually is 20V, so I won't get close to it. that's the only thing I can think of, saturation is not a problem, it's the Vgs that you need to be careful. Once you over that, the FET burn.

One option is to use transient suppressor. They have device that has low capacitance for digital data lines. Find a bidirectional 10V one.

[NateS]

Make sure you use protection diode on the gate to prevent the gate from swinging over 15V either way. Read the spec, usually is 20V, so I won't get close to it. that's the only thing I can think of, saturation is not a problem, it's the Vgs that you need to be careful. Once you over that, the FET burn.

One option is to use transient suppressor. They have device that has low capacitance for digital data lines. Find a bidirectional 10V one.

If the gate capacitance or some other factor isnt' driving selection, I like mosfets with integrated gate zeners. Particularly if I'm modifying rather than scratch building.

[Alan0354]

If the gate capacitance or some other factor isnt' driving selection, I like mosfets with integrated gate zeners. Particularly if I'm modifying rather than scratch building.

Isn't that life?!!! If you use tubes, you have exactly 12AX7 and 12AT7 to choose from...........OK 12AU7 also!!! With MOSFET, then you have to thumb through pages after pages of devices. I retired 8 years, I never even seen MOSFET with build in voltage protection.

One thing about MOSFET, the input capacitance can be high. for 12AX7 stuffs, input capacitance is around 100pF with the Miller cap in. But the advantage in your case is you have low gain, so Miller cap is small in your case. Use as small a MOSFET that can take the voltage and power ( low). The larger the MOSFET, the higher the input capacitance. You can easily get into nF for bigger HV ones.

I am not sure you can find a BJT to do it. I took a quick look in DigiKey, Beta is about 20 or max 30 at 50mA. That is way too low. You use about 3mA collector current, the base current is going to be over 100uA. that's a lot for high impedance circuit.

[NateS]

Not that it matters here, but you can find high voltage npns w decent gain. But the whole thing doesnt really need to be hv, you just need differential current....

When you wanna get down, get down close to ground.... cascode.

Ok, that was mostly for humor.

[Alan0354]

Not that it matters here, but you can find high voltage npns w decent gain. But the whole thing doesnt really need to be hv, you just need differential current....

When you wanna get down, get down close to ground.... cascode.

Ok, that was mostly for humor.

If you open to transistors, there are so many different options now. You have PNP and all.