current limiting mosfet circuits

[ant17]

hi i am currently building transmitter for a experimental project which has main driving Transistor which i want to be able to limit the current through the transistor to 12v at 6 amps which normal resistors are out as they would burn up i have thought of inductive current limiting but by frequency through inductor you would need something in the nano Henry range if it was run in megahertz range can anyone tell me what the normal way to limit the current is

 

[HMS1021]

We must know the type of transistor before we can help. A specific part number would even be more helpful. The approach for a FET is completely different that that for a bipolar

 

[ant17]

We must know the type of transistor before we can help. A specific part number would even be more helpful. The approach for a FET is completely different that that for a bipolar

its a mosfet

 

[FvM]

its a mosfet

I fear, that's the only clear point in your post - because it's written in the title.

i want to be able to limit the current through the transistor to 12v at 6 amps

Limit the current to 12V?

Seriously speaking, what's the circuit, what's the load, is it switching or linear control?

The question has been posted in the power electronics forum, but you are talking about a "transmitter" and MHz frequency...

Inductors don't limit current, just dI/dt respectively AC current. So only in a specific circuit, an inductor might act as a current liniting means.

 

[HMS1021]

I fear, that's the only clear point in your post - because it's written in the title.

Limit the current to 12V?

Seriously speaking, what's the circuit, what's the load, is it switching or linear control?

The question has been posted in the power electronics forum, but you are talking about a "transmitter" and MHz frequency...

Inductors don't limit current, just dI/dt respectively AC current. So only in a specific circuit, an inductor might act as a current liniting means.

It wasn't clear to me if the rf device or the bias circuit was a MOSFET.

Most commercial rigs simply use a resistor divider and set the gate voltage to a fixed value normally in the .6 to .8 range. R1 from Vdd to pot, pot to ground, pot wiper through R2 to the gate. R1 is set to ensure the max Vg does not exceed a safe level when the pot is cranked to the max. R2 will cause Vg to drop during input RF overdrive since Ig will increase during overdrive.

In space and more expensive systems they use the same circuit as above with a few extra zeners and ESD Clamp diodes AND they place a small resistor in the source which provides feed back providing a more constant current and thermal run away protection. This approach is really necessary if paralleling multiple devices to ensure against current hogging killing a device. The source resistor is well bypassed with caps to maintain gain and Pout.

 

[Kenneth_Potter]

Hi

Ar you aware that you can use pwer resistors and heat sinks? Perhaps you could consider using one as a degenarite source resistor.

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Hi

Ar you aware that you can use power resistors and heat sinks? Perhaps you could consider using one as a degenarite source resistor.

 

[HMS1021]

Using a source resistor is very good from a bias method. Vgate-GND is fixed, Id increases, drop across Rsource increases, Vgs decreases reducing the current and it is faster than any thermal run-away. But there is a price: The thermal path from source to ground is critical. There are a dozen ways to maintain thermal but they are all extra work and parts.

 

[ant17]

Using a source resistor is very good from a bias method. Vgate-GND is fixed, Id increases, drop across Rsource increases, Vgs decreases reducing the current and it is faster than any thermal run-away. But there is a price: The thermal path from source to ground is critical. There are a dozen ways to maintain thermal but they are all extra work and parts.

hi i will try and be more clearer it 555 timer circuit running at 100khz square wave driving the gate through a 10 ohm resistor the with source connected to ground i want antenna connected at the drain and want apply 12 volts to the drain but i want to limit the through the drain to 6 amps or 72 watts

 

[FvM]

Looks like you are asking about a suitable DC current path for the amplifier output. Perhaps you should read about class C/E/F RF power amplifiers.

I guess, no edaboard member likes to see the said "experimental project" in his neighbourhood or would be motivated to support it...