How closely do the recommended Power up/down sequence have to be followed. Can the HMC8410 be turn on and off by the applying -2V/-0.5V to VGG1?
Yes, the HMC8410 could be turned "on" and "off" by toggling Vgg1 between two voltages. With a typical HMC8410 the target Iddq = 65 mA would be obtained using Vgg1 = -0.55 V (approximately) and the drain would be considered as pinched-off using Vgg1 = -2.0 V. However, there will be some lot-to-lot and device-to-device variability in those values. In other words, across all devices -2.0 V might not always result in total pinch-off and -0.55 V might not always result in exactly 65 mA.
The power-up/power-down bias sequences are provided mainly to ensure that the part does not suffer excessive stress during biasing. Manual (on the bench) biasing is the situation in which such stresses seem the most likely to occur. The sequences do not need to be strictly followed but caution should still be exercised. We have not evaluated the survivability of the part under heavy RF drive and without DC bias...so we wouldn't recommend having the part under heavy RF drive without DC bias applied. Also, the reason for the sequence between gate and drain bias is to ensure that the drain is biased only with the channel under gate control and not with the channel wide open. Applying Vdd while the gate is open or set to too positive of a voltage could result in excessive drain current, Pdiss, and possibly damage to the device. If your drain supply offers protection so that such stresses do not occur, then the exact gate-to-drain sequencing becomes less important. As an example of safely violating the sequence from the data sheet, you could have a small RF signal present at RFIN before the gate and drain biases have been applied, then apply a pre-determined gate voltage of -0.55 V (rather than starting with the gate at -2.0V and then increasing it to the target gate value), then apply a drain voltage of 5V...and everything should be fine.
I moved your question regarding the HMC8410 to the RF and Microwave community. Someone here should be able to assist you.
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