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> Re my H Bridge cct that I posted earlier today.
>
> Critiques, or even scathing criticism, if objective, are most 
> welcome.

Let me start by criticising my own design :-)
Things are usually not as straight forward as they seem.

The upper and lower driver transistors Q2 & Q3 share the same base 
current which flows via Q1.
The current source adjusts Q3 base current until Q3 will just support 
the requisite current. The voltage drop across Q3 alters to make this 
happen depending on battery voltage. BUT Q2 has the same base current. 
IF the two transistors were matched this would be a form of current 
mirror and the voltage drop across Q2 would be about the same as 
across Q3. As they are similar transistors it will be similar.

This is not necessarily bad as, arguably, the circuit "doesn't care" 
where excess voltage is dropped to make the feedback work properly. In 
practice I'm not sure what the second order effects are. A closer look 
is needed to make sure it doesn't cause problems.

Taking the bottom of R4 to Q3 collector and taking Q1 emitter to 
battery may seem to be a way of preventing the above and causing Q2 to 
saturate when driven - in practice it causes oscillations. More 
thinking required ...



        RM

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