The use of an adequate gate
bias resistor was shown to be useful in producing a wide linearity sweet-spot in junction FET devices.
An innovative amplifier design has been developed that allows traditional medium power Darlington feedback amplifiers to operate directly from 5 V without the need for an external
bias resistor. The patent-pending design has been applied to the Sirenza Microdevice 5 V InGaP SBB gain block product series.
The detector diode operates under very high
bias resistor; therefore, [eta][V.sub.T] [much less than] [I.sub.d][R.sub.b].
The control voltage [V.sub.c] is applied to the diodes through the
bias resistor [R.sub.b] and a [lambda]/4 transmission line.
The base
bias resistor must take this voltage drop into account.
By placing the
bias resistor [R.sub.b] close to the feed line, the forward transmission coefficient |[S.sub.21]| has a broadband response and the RF leakage signal is minimized over a wider range of frequencies.
The only minor modification to the high pass/low pass 22.5[degrees] bit was the addition of a 1 to 2 k[ohm]
bias resistor from the PHEMT drain to ground to avoid a floating bias potential.
For systems where the ambient temperature does change, an optional ambient temperature
bias resistor is available, which balances out the effects of changing temperature.
This time constant is minimized by using the smallest FET device geometry and by setting the gate
bias resistor to the lowest value without degrading isolation.
If necessary, these times can be reduced to 15 ns by reducing the
bias resistor to 1 k[ohms].
To ensure minimum gain change over the operating temperature, a voltage drop of 2 V minimum is recommended for the
bias resistor.