# feedback techniques

#### lean

Joined Oct 15, 2004
3
Hello everybody. I would like ask you for a curious phenomemnon by
measuring
the output impedance in a circuit. Take a look to this
(www.eurobotics.org/vco/circuito.doc) simple circuit. When I meassure
the
output impedance using the convencional way, that is, using a voltage
source
at the output while shortcutting the dependences ones I get 6 MOhmns. If
instead of this I use feedback techniques to calculate the output
impedance I
get 30 GOhmns... What is going on?.

If transistor is a Mosfet.
When I meassure the output impedance using the convencional way, that is, using a voltage source
at the output while shortcutting the dependences ones I get 60 GOhmns. If
I use feedback techniques to calculate the output
impedance I
get 60 GOhmns.
when I work with BJT the values are different.
What happen with rpi and ru?

Thank you all.

#### pebe

Joined Oct 11, 2004
626
Originally posted by lean@Oct 15 2004, 12:59 PM
Hello everybody. I would like ask you for a curious phenomemnon by
measuring
the output impedance in a circuit. Take a look to this
(www.eurobotics.org/vco/circuito.doc) simple circuit. When I meassure
the
output impedance using the convencional way, that is, using a voltage
source
at the output while shortcutting the dependences ones I get 6 MOhmns. If
instead of this I use feedback techniques to calculate the output
impedance I
get 30 GOhmns... What is going on?.
The circuit shown on your link is an op-amp with its output fed back to its inverting input via an emitter follower. So Voutput = Vinput - approx 0.7V
How is this used to measure output impedance? and of what?
If transistor is a Mosfet.
When I meassure the output impedance using the convencional way, that is, using a voltage source
at the output while shortcutting the dependences ones I get 60 GOhmns. If
I use feedback techniques to calculate the output
impedance I
get 60 GOhmns.
when I work with BJT the values are different.
What happen with rpi and ru?

Thank you all.
[post=2944]Quoted post[/post]​
Can you explain how you measure output impdance 'using conventional way' and 'using feedback techniques'?

#### lean

Joined Oct 15, 2004
3
In Mosfet:

When I meassure the output impedance using the convencional way,

Vx/ix= RE +ro(1 + gmRE(1+Ad))= 60Gohmios

If I use feedback techniques to calculate the output impedance,
I have to calculate zoA,

and Zof= zoA(1 + A*f)

because Io= gm(AdVi - IoRE) -IoRE/ro

zoA= V2/I2= RE + gmREro + ro = 6.064Mohmios

f=RE

Zof= 60Gohmios

#### pebe

Joined Oct 11, 2004
626
Originally posted by lean@Oct 17 2004, 11:37 AM
In Mosfet:

When I meassure the output impedance using the convencional way,

Vx/ix= RE +ro(1 + gmRE(1+Ad))= 60Gohmios

If I use feedback techniques to calculate the output impedance,
I have to calculate zoA,

and Zof= zoA(1 + A*f)