> There are some micro RC circuits that are using op-amps as H-bridge
> drivers.  I was surprised, but apparently some rail-rail opamps have
> surprisingly high current capabilities.  The MAX4020 has 4 opamps
> and drives +/-120mA;

Nice IC. But alas almost any opamp is going to have a quiescent 
current that necessitates adding a series control element as well. 
And anything from Maxim is expensive. They make some nice products but 
yoy tend to pay a premium for any of their parts that aren't second 
sourced. The LM324 / LM358 are cheap enough but have nowhere near the 
required drive. And Iq is OK for many purposes  but not for this.

> Audio amps are another possibility.  the BEAM robotics people have
> used LM386 amps as motor drivers in a number of applications.

I like the LM386. Probably over $US.20 each. Quiescent will need an 
extra switch.
Don't recall the ioperating supply range offhand or max power or 
saturation voltage. Any one could trip it up, but audio amps as a 
whole are a good idea.

> (I suspect that at 4V supply, your sorta-darlington configuration
> suffers a lot from the inherent high Vcesat of a darlingtons.  You
> could be looking at as much as 1.2V on each side of the load, right?

Yes, in original circuit.. Bottom stage drops 1 x Vbe plus transistor 
saturation.
Top driver the same PLUS base bias drop across drive rsistor - about 1 
volt extra. Combined it's 2.5V+ :-(

> (I'm not sure both sides are exactly darlingtons...)  If that's the
> case, you could get some major benefit by replacing the input 
> transistors
> with fets?)

Yes. Or by rearranging the circuit so bridge switches are common 
emitter rather than common collector. Bipolar have some other 
advantages here, apart from being readil;y available.




            Russell McMahonl


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