Quote:
Originally Posted by old_coot88
Since the non-inverting output feature is not needed, it could be eliminated. This would allow the output level to take advantage of the full supply voltage (rather than half as it is).
It's always useful to think of a transistor as a triode analog - with the emitter the cathode, the base the grid, and the collector the plate.
In the sketch shown, start out with R2 as 4.7K, and R3 as 1K. Then for R1, first try 100K and work down in value till you get 0.7 V on the base. Once a usable output is obtained, then play with values for R2 and R3 and see if the output level can be improved any, all the while tweaking R1 to maintain 0.7V on the base.
Edit.. Error in sketch, 1K resistor "R2" should be R3. Dumb, dumb.   |
This circuit will not work as a decent video amplifier because there is no emitter resistor. The gain will be very large and very dependent on bias - in other words, without an emitter resistor, you have a switching circuit rather than a linear amplifier. EDIT: this has sometimes been used as a sync separator circuit - a very non-linear switch that changes state on sync pulses.
If you want to have both inverted and non-inverted outputs (although with gain = 1), go back to the circuit with equal emitter and collector resistors and make the upper base resistor about 15k or 18k as I suggested. If you want non-inverting with gain, use the feedback circuit suggested by kf4rca. If you need inverting with gain, you can used the single transistor circuit, but the emitter resistor needs to be smaller and the base bias voltage needs to be reduced so the collector voltage is at the middle of its possible range.
Second edit: I forgot to add that the gain for the inverted signal on the collector is close to the ratio of the collector resistor divided by the emitter resistor. The gain of the non-inverted signal on the emitter is always (very) slightly less than 1.