W
Warren
- Jan 1, 1970
- 0
I see the common emitter configuration like the following
frequently:
http://imageshack.us/photo/my-images/828/groundedemitter.png
This is not a configuration I would normally use, but I would
like to understand it better. It seems to appear frequently in
60s/70s era designs.
To bias this, you take the ratio of R3 to R2, to arrive at a
collector voltage (target usually about half of Vcc). This
part is easy- in this example the ratio is 10:1. So the
resulting collector voltage is approx (10 + 1) times Vbe,
giving about 7 volts.
The link above is a LTspice simulation using 2N3904, with
a DC Beta of 300.
QUESTIONS:
(I only care about audio frequencies here)
Since there is negative feedback involved:
1. How do you compute the small signal gain?
2. Input impedance?
3. Output impedance?
LTspice output of a small AC wave form showed a gain
of about 219 (9.9mV amplitude in, 2.165V amplitude out).
But with Beta at 300, I would have thought the gain to
be near 300 minus the signal feedback (1/10th),
which would be about 270. But the simulation showed about
219 instead.
I'm guessing the input impedance to be approx R2||R3+R1
(nearly just R2). But with the negative fb, I suspect that the
input impedance is raised considerably.
I'm also guessing the output impedance to be approx R1||R3+R2
(nearly just R1).
Thanks, Warren
frequently:
http://imageshack.us/photo/my-images/828/groundedemitter.png
This is not a configuration I would normally use, but I would
like to understand it better. It seems to appear frequently in
60s/70s era designs.
To bias this, you take the ratio of R3 to R2, to arrive at a
collector voltage (target usually about half of Vcc). This
part is easy- in this example the ratio is 10:1. So the
resulting collector voltage is approx (10 + 1) times Vbe,
giving about 7 volts.
The link above is a LTspice simulation using 2N3904, with
a DC Beta of 300.
QUESTIONS:
(I only care about audio frequencies here)
Since there is negative feedback involved:
1. How do you compute the small signal gain?
2. Input impedance?
3. Output impedance?
LTspice output of a small AC wave form showed a gain
of about 219 (9.9mV amplitude in, 2.165V amplitude out).
But with Beta at 300, I would have thought the gain to
be near 300 minus the signal feedback (1/10th),
which would be about 270. But the simulation showed about
219 instead.
I'm guessing the input impedance to be approx R2||R3+R1
(nearly just R2). But with the negative fb, I suspect that the
input impedance is raised considerably.
I'm also guessing the output impedance to be approx R1||R3+R2
(nearly just R1).
Thanks, Warren