Common-Emitter output resistance

[jack23]

Hello all,

I have a problem that has been assinged to me. It is involving a Common-Emitter Amplifier. Please see attached PDF. I have looked over my texts and can't seem to find a way to get started in tackling this problem. Any feedback would be GOLD! Thank you.

 

[audioguru]

The common-emitter transistor is a current-sink with an extremely high output impedance. The output impedance of the entire amplifier circuit is basically only the value of the collector resistor.

 

[Carl-NC]

Ugh... are people still using h-parameter notation?

While a degenerated CE stage has a high output resistance, it still has an output resistance. I believe the question is, "What is the output resistance at the collector, ignoring RC?"

To solve this, imagine RC to be infinite; now apply a "wiggle voltage" to eo. When you wiggle eo, how much current will flow into (or out of) the collector? The "Show That" equation gives you a hint: hoe. What is hoe? How can it be represented using a physical element? Draw that element into the diagram, and start drawing and labeling where currents might flow due to wiggling eo.

- Carl

 

[audioguru]

The schematic doesn't show a bootstrapped collector resistor which increases the output impedance of a common-emitter amplifier dramatically.

 

[Carl-NC]

The schematic doesn't show a bootstrapped collector resistor which increases the output impedance of a common-emitter amplifier dramatically.

True, it's an internal characteristic of the transistor. And, actually, the ro of the transistor decreases the output impedance. That is, a device with an (ideal) infinite ro will have an infinite output impedance.

- Carl