Bob,
I appreciate your prior help and I am coming to you again for assistance.  =
I am still trying to use your H-bridge design (with very few changes).
I have exchanged the 2n2222 switching transistors with 2n2102 and I have =
exchanged the TIP120/125 with TIP140/145.

The 2n2222 have a breakdown voltage of 40VDC and the 2n2102 have a =
breakdown voltage of 65VDC.  I am using the H-bridge with a 48VDC geared =
motor (at 300Hz and a max of 450rpm).  currently I can easily control a =
12VDC motor from 0-100uty.  When I increase the voltage and use a 48VDC =
motor the 2n2102 transistors heat up and my scope shows very little change =
in speed when the duty cycle is changed.  I have simulated the circuit and =
all component voltages are within spec.  I am not sure where to go from =
here.  Will biasing the circuit differently be useful (or is there any =
other easy fix)?
Thanks for your time,
Tom=20

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