# Bias up and down sequence of a GaN power amplifier

I am trying to build a biasing sequence circuit for a GaN power amplifier.

The GaN power amplifier I am using has a particular bias-up and bias-down sequence as shown below:

The figure below is a part of a GaN power amplifier sequencing circuit:

Here I am trying to switch the drain of the power amplifier. The PULSE_IN signal goes to the drain of the power amplifier. Initially the POT(VR1) is set such that VG1_REF is 5 V .

DURING TURN-ON:

The VG1_REF signal (5 V) goes to an inverting amplifier and the output of that inverting amplifier (-5 V) is given to the gate terminal of the PA which sets the Gate of the PA in pinch-off first. For that to happen R17 is grounded using Q4 (Initially the ENABLE_1 signal is given which would be a constant high. That will turn-on the Q4 and R17 will be grounded).

Next, I will give the PULSE_IN signal to the drain of the power amplifier.

Next, I will send a constant low signal from ENABLE_1 so that Q4 will be turned-OFF and I will adjust the POT (VR1) to set the VG1_REF (Gate voltage to approx. ~ -2.2 V) to get my required drain current

And finally I will give my RF input signal.

DURING TURN OFF:

First I will remove my RF input signal

Next, I will reset the POT (VR1) such that VG1_REF will be 5 V and send constant high from ENABLE_1 to turn ON Q4 so that a pinch off voltage (-5 V) is applied to the Gate terminal of the PA.

Then I will turn-off the PULSE_IN signal (to turn-off Vd).

Then I will finally turn-Off ENABLE_1.

I want to control the constant voltages from ENABLE_1 and the pulsed signals from PULSE_IN using a single Control circuitry. So, please suggest how can I control ENABLE_1 and PULSE_IN together according to the logic I explained above.

To sum-up the required logic:

During TURN-ON

1. ENABLE_1 -> Constant high
2. PULSE_IN -> Pulse input
3. ENABLE_IN -> low to adjust the POT (while PULSE_IN not turned-off)
4. RFIN -> given to PA

DURING TURN-OFF:

1. RFIN -> TURN OFF

2. ENABLE_1 -> CONSTANT-HIGH

3. PULSE_IN -> OFF

4. ENABLE_1 -> OFF