DC Characteristics of MOSFETs in amp


I have been considering all along that I was going to operate the MOSFETs at a higher voltage and enjoy the lower drain current and get more gain from the RD100HF1 devices. I thought this because it worked so well with the IRF510 MOSFETs.

Tonight, when I tried to use 24VDC drain voltage, I did not realize my expectations.  My 24V test did not exhibit the power out I expected and the current was excessive. The devices were obviously targeted for mobile operation at 12V. An external battery booster may be an option to keep the voltage around 13.5V. My search for documents online confirming or denying the possibility of higher drain voltage operation turned up empty.

 VDD-PO Characteristic2

This curve from the Mitsubishi spec sheet shows the performance up to 14VDC .

I am very cautious in measurements as I begin to explore the amp performance. I designed in a 0.01 ohm shunt resistor on the board to test points for measuring this voltage. As a point of comparison, the FLUKE meter measured 0.24A in idle and the shunt measured 0.23A. The relationship is 1mV measured at the shunt test points = 100mA of drain current.

To begin, I set the bias current to 100mA per MOSFET. I want to eventually set it to 1A per MOSFET when I am able to remove the drain voltage while not in transmit. An electronic switch/circuit breaker will do this job. Right now, all measurements and controls are manual. I have inline a 4dB resistive pi-network selected. Here is the tabulated data at 100mA bias. holding the frequency constant and varying the power in. I was using my K2 power adjust knob to set the power in.

The chart is not cooperating so the formatting is not what I want it to be.

73, K5OOR –  Virgil


PO Curve





V in    13.5   Date 11/30/2012  
Time 0:55  
FREQ P1 P2 A DC DC W Efficiency P2/P1 dB Gain
4 2 38 10.0 135.0 28.1 19 13
4 3 50 11.9 160.7 31.1 17 12
4 4 70 13.5 182.3 38.4 18 12
4 5 80 13.9 187.7 42.6 20 13


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