Hi,
I'm trying to make a low power emitter follower (so that output matches input) that can be powered by a larger voltage than the supported Vce (see the attached images for the circuit) while still having a fast response time.
The transistor is actually a phototransistor. Initially I tried connecting a resistor between Vcc and the collector followed by a zener (~5.2v) between collector to ground. This ensures that Vce doesn't exceed 5.2v. Then there's a load resistor between emitter and ground. The output is measured at the emitter. In order to get a good frequency response I tried using as small a load resistor as possible.
The problem with this circuit, is if the resistor is too small (such as in the image), the load resistor will form a resistive divider with the collector resistor so that in this instance (Rc= 100k, Re= 30k) it takes a Vcc of 24v for the output to reach the required 5.2v (see graph simulated with pspice).
My second thought was to replace the load resistor with a similar zener which seems to resolve the above issue (see graph).
My question is if this is a good idea and what are the potential pitfalls that I might not know about. What would the frequency response be like for this circuit? I don't have zeners on hand to test this circuit.
Thanks in advance,
M
EDIT: I think the load zener should have a rated zener voltage that is less than the zener voltage of the other zener by at least Vce-sat.
I'm trying to make a low power emitter follower (so that output matches input) that can be powered by a larger voltage than the supported Vce (see the attached images for the circuit) while still having a fast response time.
The transistor is actually a phototransistor. Initially I tried connecting a resistor between Vcc and the collector followed by a zener (~5.2v) between collector to ground. This ensures that Vce doesn't exceed 5.2v. Then there's a load resistor between emitter and ground. The output is measured at the emitter. In order to get a good frequency response I tried using as small a load resistor as possible.
The problem with this circuit, is if the resistor is too small (such as in the image), the load resistor will form a resistive divider with the collector resistor so that in this instance (Rc= 100k, Re= 30k) it takes a Vcc of 24v for the output to reach the required 5.2v (see graph simulated with pspice).
My second thought was to replace the load resistor with a similar zener which seems to resolve the above issue (see graph).
My question is if this is a good idea and what are the potential pitfalls that I might not know about. What would the frequency response be like for this circuit? I don't have zeners on hand to test this circuit.
Thanks in advance,
M
EDIT: I think the load zener should have a rated zener voltage that is less than the zener voltage of the other zener by at least Vce-sat.
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