Calculation of input impedance (Common emitter amplifier)

jazko · 20th January 2011, 02:27 PM

Hello everyone, can someone give me a clue how to calculate the input impedance of the circuit shown? β=100.

Thanks

sawreyrw · 20th January 2011, 02:46 PM

The input resistance of the transistor is Rpi=B/gm, where gm is the transconductance and is given by gm=40*Ic. Ic is the DC collector current.

alexcp · 03:26 PM

Be careful with the units; it is 40 (1/V) in the formula, so if you take Ic in amps, you get Gm in 1/ohm and Rpi in ohm. Milliamps will give you kiloohms, etc. You need to add R1 and R2 in parallel to the input resistance of the transistor to get the input impedance of the circuit.

AndrewT · 04:00 PM

combining those two replies
Input impedance = R1//R2//[Rpi=B/gm]

R1=10k seems ultra low in value cf. R2=82k

Should there be a term to represent the effect of Re//Rc has on the input impedance?

DF96 · 20th January 2011, 03:58 PM

Homework?

SY · 20th January 2011, 04:06 PM

Quote:

Originally Posted by DF96

Homework?

Looks that way. And fortunately, no one has given a fully correct answer.

hint: the answer is frequency dependent...

AndrewT · 20th January 2011, 04:11 PM

Quote:

Originally Posted by AndrewT

Should there be a term to represent the effect of Re//Rc has on the input impedance?

Quote:

Originally Posted by SY

hint: the answer is frequency dependent...

Rc should give that frequency dependence.

Unless you mean the effect of the DC blocking capacitor.

DF96 · 20th January 2011, 04:28 PM

What about Early effect? That will change the effective value of Rc.

Should we ask whether the OP is looking for a first-year or second-year answer?

audiohead · 20th January 2011, 04:36 PM

Zin = R1||R2|| hie.

Art

wakibaki · 20th January 2011, 05:05 PM

Quote:

Originally Posted by SY

hint: the answer is frequency dependent...

We're only interested in the range where the amplifier is going to be operated, so other than calculating and noting the lower cut-off frequency for all practical purposes the value can be taken as R1||R2||hfe*re.

Quote:

Originally Posted by DF96

Should we ask whether the OP is looking for a first-year or second-year answer?

No. We should expect that if the OP knew the first-year answer he would have couched the question to show that.

w

20th January 2011, 03:18 PM	#3
alexcp diyAudio Member Join Date: Jun 2009 Location: Florida Blog Entries: 35	Be careful with the units; it is 40 (1/V) in the formula, so if you take Ic in amps, you get Gm in 1/ohm and Rpi in ohm. Milliamps will give you kiloohms, etc. You need to add R1 and R2 in parallel to the input resistance of the transistor to get the input impedance of the circuit. Last edited by alexcp; 20th January 2011 at 03:26 PM.

20th January 2011, 03:56 PM	#4
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	combining those two replies Input impedance = R1//R2//[Rpi=B/gm] R1=10k seems ultra low in value cf. R2=82k Should there be a term to represent the effect of Re//Rc has on the input impedance? Last edited by AndrewT; 20th January 2011 at 04:00 PM.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
OT primary impedance calculation	cb951303	Tubes / Valves	14	12th April 2013 04:18 AM
Input impedance resistor, F4 amplifier	Manolo47	Pass Labs	7	5th February 2011 05:05 PM
Basic Common Emitter Amp Help	Obe1	Analog Line Level	99	21st February 2010 05:08 PM
How To Measure Input Impedance Of Solid State Amplifier?	kelticwizard	Solid State	11	28th January 2006 03:34 PM
measuring Input impedance of an Amplifier	pinkmouse	Solid State	5	23rd May 2002 08:15 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

20th January 2011, 02:46 PM	#2
sawreyrw diyAudio Member Join Date: Apr 2006 Location: Minnesota	The input resistance of the transistor is Rpi=B/gm, where gm is the transconductance and is given by gm=40*Ic. Ic is the DC collector current.

20th January 2011, 03:58 PM	#5
DF96 diyAudio Member Join Date: May 2007	Homework?

20th January 2011, 04:28 PM	#8
DF96 diyAudio Member Join Date: May 2007	What about Early effect? That will change the effective value of Rc. Should we ask whether the OP is looking for a first-year or second-year answer?

20th January 2011, 04:36 PM	#9
audiohead diyAudio Member Join Date: Mar 2005 Location: Los Angeles, california	Zin = R1\|\|R2\|\| hie. Art