RussellMc wrote: > > D) "Interesting" [tm] effects can occur from ringing and transient > oscillations between FET gate capacitances and lead inductances. Worst > case these also lead to magic smoke. A reverse biased (usually) gate > to source zener mounted as close to the FET gate-source terminals as > possible can help muchly. This should have a V rating > max gate drive > voltage applied, < Vgs max. Dissipation usually low. A Gate to source > reverse biased Schottky diode is sometimes recommended here to quench > oscillations but does not act asn an overvoltage gate clamp as a zener > does. Some say that zeners can lead to gate oscillation BUT I have had > extremely good results with them. Circuits which would typically > achieve a gate source punch through in minutes of running have worked > well indefinitely with a zener present. This also protects (as above) > against high voltage transients connected to the gate via the d-g > Miller capacitance. (see cited article). > > This is an interesting subject, esp. if you are trying to make a linear RF amplifier using only TO220 or larger SMPSU package transistors. One approach is a step down transformer to 12 Ohms or even 6 Ohms from 50 Ohm driver stage. Also the Gate Drain capacitance changes a lot with supply volts If everything is not done proper you can get a nasty peak in response with evil phase shifts just before it rolls off. It seems possible to use 300W 900V 10A 1200pF gate capacitance SMPSU parts at up to maybe 2MHz or 4MHz. For 50MHz you need much lower power, lower current types. IRF510 etc.. An IRF740 isn't too bad. If you think this is a strange occupation, check out price of 150W HF FETs intended for RF. (Easily 50x price!). 50MHz or maybe 70MHz is possibly the limit for creative use of $1 FETs. For 100MHz to 1000MHz you need the big flat leads for low inductance, beryllium oxide insulating substrate for heat sink, specialist and easier to break RF optimised chip design etc. The magic smoke is very easily released from thes parts. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist