converting positive to negative voltage

Thread Starter

jsnx

Joined Mar 17, 2022
21
Hello, I'm new to the analog design world and I have a probably dumb question.

I have a normally-on transistor that I want to shut off with a negative voltage. The issue is that I only have a positive supply voltage and so I thought that I can generate the negative voltage from GND by using a negative charge pump with a voltage regulator system.

Here comes the dumb question; can't I just invert the positive supply voltage using an inverting amplifier and fed it to my gate driver that controls the transistor? Does it have any drawbacks? Would it be bad for my transistor?

Thank you.
 

ronsimpson

Joined Oct 7, 2019
2,062
can't I just invert the positive supply voltage using an inverting amplifier
Without a schematic it is hard to see what you are doing. An inverting amplifier with an output that goes below ground will need a negative supply.

How do you have a transistor that is normally on?
 

BobTPH

Joined Jun 5, 2013
4,897
You must be talking about a jfet, right? They are uually biased by raising the source voltage so that the gate can have a negative voltage with respect ro the source.

Bob
 

crutschow

Joined Mar 14, 2008
29,779
The issue is that I only have a positive supply voltage and so I thought that I can generate the negative voltage from GND by using a negative charge pump with a voltage regulator system.
Yes, that's one relatively simple way to generate a negative voltage from a positive supply.
There are a number if IC's that can do that, or you could use a discrete circuit, such as ones that use a 555 oscillator.
 

Thread Starter

jsnx

Joined Mar 17, 2022
21
Without a schematic it is hard to see what you are doing. An inverting amplifier with an output that goes below ground will need a negative supply.

How do you have a transistor that is normally on?
Thank you for your response, it a normally on HEMT used in a power amplifier and I need to apply a negative voltage to deplete the channel and turn it off
 

Thread Starter

jsnx

Joined Mar 17, 2022
21
You must be talking about a jfet, right? They are uually biased by raising the source voltage so that the gate can have a negative voltage with respect ro the source.

Bob
Thank you for your response, it's a HEMT and I can not control the source voltage since it is used in a power amplifier
 

Thread Starter

jsnx

Joined Mar 17, 2022
21
Yes, that's one relatively simple way to generate a negative voltage from a positive supply.
There are a number if IC's that can do that, or you could use a discrete circuit, such as ones that use a 555 oscillator.
Thank you for you reply, I'll check this kind of circuits
 

Papabravo

Joined Feb 24, 2006
18,408
I have a 5V supply voltage and I need a -5V to turn off the HEMT transistor
An inverting buck-boost converter can provide the required inversion and with more available current and better regulation than a charge pump.

EDIT: It might be the case that if you want -5V or lower negative voltage, that you have to start with a larger positive voltage. So, if you start with a boost converter from +5 to +8 for example and then go from +8v to -5v, that will work better for you.
 
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ronsimpson

Joined Oct 7, 2019
2,062
What is the timing on the Gate Drive signal? Example: PWM 10 to 80% 100khz or On for 10 minutes and off for 3 minutes?
What it the part number of the HEMT transistor? I am driving to -1V/+6V.
 
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