Electronic Design

GaAsFET Power Supply Has Negative Gate Bias, Automatic Power-Up Sequencing

GaAsFETs are frequently used as high-power RF amplifiers for cell-phone repeaters and similar devices. In general, they require a negative supply voltage of −4 V to −5 V to bias the gate of the FET. This must be a low-ripple supply so as not to introduce sidebands in the RF spectrum. GaAsFETs also require that the −5-V bias supply be close to the regulated regulated level before the drain voltage, usually around 10 V, is applied to the FET. If this requirement isn’t met, a new FET would have to be purchased. The circuit shown satisfies both of these requirements (see the figure).

Operation is as follows: U2, an LT1373 monolithic switching regulator IC, is configured in a Cuk converter topology to generate −5 V from a 10-V input supply. The advantage of the Cuk topology for this application is its very low inherent output ripple. The LT1373 has a negative feedback (NFB) pin, pin 3, that makes regulating a negative output very simple.

Q3 turns the system on or off. With a logic high on Q3’s gate, current flows through the base of Q1, which, in turn, clamps the gate of p-channel FET Q2 to its source. Q2 is therefore off, and the 10-V supply to the GaAsFET is turned off. U2’s shutdown pin (S/S) is grounded by the drain of Q3, forcing the −5-V regulator off. When the gate of Q3 is pulled low, U2 turns on and the −5-V supply starts.

The power-up sequencing is handled by U1, an LT1431 shunt regulator. U1 has its ground referenced to the −5-V rail. When the −5-V supply reaches approximately −4.5 V, the LT1431’s reference pin is pulled 2.5 V above its ground pin. The pin is at −5 V by the divider, which consists of R5 and R6. U1’s open collector, pin 1, pulls low at that point, which forces the gate of Q2 low, turning on Q2 and applying power to the GaAsFET drain.

If an “off” command is later received by Q3, the result is to force Q1 on, shorting the gate of Q2 and immediately turning off the drain supply. The negative regulator is turned off at the same time, but the negative supply’s load is very light. Therefore, the droop rate of this supply also is very low. This ensures that the gate supply discharges after the drain supply.

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