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Problem with emitter follower bootstrapped.

F

Francesco

Jan 1, 1970
0
Hi, I have a problem with this circuit, I don't understand why the input
resistance is Vi/Ir, why should omit the current through the base of
transistor? I think the input resistance should be Vi/(Ir+Ib). on DC
level, the current through the voltage divider is much larger than the
base current, but on AC level, since now we have a bootstrap in the
circuit , the bootstrap resistor becomes(or at least looks like) larger
and i think it is incorrect to neglect the base current.

of course, as you could see, there must be some incorrectness in my
mind, plz do help me overcome it. Thanks.
 
K

Ken Smith

Jan 1, 1970
0
Hi, I have a problem with this circuit, I don't understand why the input
resistance is Vi/Ir, why should omit the current through the base of
transistor? I think the input resistance should be Vi/(Ir+Ib). on DC
level, the current through the voltage divider is much larger than the


Since you are, I assume, after the AC input impedance, you really need to
look at the change in input voltage vs the change in input current.

You are right that the base current of the transistor matters. If you
have the DC bias network bootstrapped the base current becomes a
significant part of the impedance. The fact that the emitter follower's
gain is not really 1.0 also matters.

If you really want high impedance, you need to feed the emitter with a
high impedance such as a constant current source.
 
F

Fred Bloggs

Jan 1, 1970
0
Francesco said:
Hi, I have a problem with this circuit, I don't understand why the input
resistance is Vi/Ir, why should omit the current through the base of
transistor? I think the input resistance should be Vi/(Ir+Ib). on DC
level, the current through the voltage divider is much larger than the
base current, but on AC level, since now we have a bootstrap in the
circuit , the bootstrap resistor becomes(or at least looks like) larger
and i think it is incorrect to neglect the base current.

of course, as you could see, there must be some incorrectness in my
mind, plz do help me overcome it. Thanks.

You are misunderstanding the text or there is a typo. The input
impedance for an emitter follower can never be less than hie+(hfe+1)Re
where Re is the emitter load. The DC base bias network will reduce the
input impedance by shunting current in parallel with the base drive. The
purpose of the bootstrap is to eliminate this shunt effect at ac signal
frequencies by driving the bias network in parallel with the load. This
makes the new input impedance hie+(hfe+1)Re' where
Re'=Re||Rbias-network. In practice, this is not perfect and the
impedance is smaller than this by a factor of 1+hie/Rbootstrap, but it
can be very close.
 
F

Fred Bloggs

Jan 1, 1970
0
The input
impedance for an emitter follower can never be less than hie+(hfe+1)Re...

That should be : can never be *more* than hie+(hfe+1)Re...
 
F

Francesco

Jan 1, 1970
0
Fred Bloggs wrote:
....cut...
You are misunderstanding the text or there is a typo.

It must be a misunderstanding :-(
The input
impedance for an emitter follower can never be more than hie+(hfe+1)Re
where Re is the emitter load.
The DC base bias network will reduce the
input impedance by shunting current in parallel with the base drive.

Well, I don't understand even this ! Why the emitter follower's input
impedance will reduce if the bias network impedance isn't much less than it?

I'm not secure of my english! If there are some error, please correct
me. Thank!
 
I

Ian Bell

Jan 1, 1970
0
Francesco said:
Fred Bloggs wrote:
...cut...

It must be a misunderstanding :-(


Well, I don't understand even this ! Why the emitter follower's input
impedance will reduce if the bias network impedance isn't much less than
it?

because the bias network is in parallel with it.

Ian
 
F

Francesco

Jan 1, 1970
0
Ian Bell wrote:
[cut]
because the bias network is in parallel with it.

Obviously the input impedance of the emitter follower load the bias
network!
But the input impedance of the amplifier is Zi = (hfe+1)Re!
How can it change this value?
 
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