Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

An old exam problem related to an amplifier

Status
Not open for further replies.

Wow_01

Newbie
Joined
Sep 19, 2019
Messages
5
Helped
0
Reputation
0
Reaction score
0
Trophy points
1
Activity points
54
Hi,

I'm trying to solve an old exam problem, which i've attached below, and need to know if I've done it correctly (I don't think I have since I've not used r0 as stated I should in the problem). Could someone please help me with this, I would really appreciate it :smile:

Problem:
Screenshot 2020-05-12 at 10.47.37.png

My solution:
IMG_4282.jpg

Thanks in advance!
 

albbg

Advanced Member level 4
Joined
Nov 7, 2009
Messages
1,256
Helped
434
Reputation
868
Reaction score
389
Trophy points
1,363
Location
Italy
Activity points
9,491
It seems correct to me (in you solution R1=RG, R2=RA), but you should also include r0. This is simply a resistor placed between D and S, then is in parallel with RD and RL.
 

Wow_01

Newbie
Joined
Sep 19, 2019
Messages
5
Helped
0
Reputation
0
Reaction score
0
Trophy points
1
Activity points
54
I don't know why I wrote R1 and R2.
Ok, nice. Thanks for the help!
 

Audioguru

Advanced Member level 5
Joined
Jan 19, 2008
Messages
9,327
Helped
2,149
Reputation
4,292
Reaction score
1,975
Trophy points
1,393
Location
Toronto area of Canada
Activity points
58,709
The input impedance is not RG//RA (it is much higher) because RG is bootstrapped with positive feedback from RA.
 

albbg

Advanced Member level 4
Joined
Nov 7, 2009
Messages
1,256
Helped
434
Reputation
868
Reaction score
389
Trophy points
1,363
Location
Italy
Activity points
9,491
Due to the presence of the (ideal) infinite capacitance capacitor, in parallel with Rs, all frequencies but DC are shunted to ground. Then there is no positive feedback through RA
 

schmitt trigger

Advanced Member level 5
Joined
Apr 17, 2013
Messages
2,727
Helped
859
Reputation
1,722
Reaction score
848
Trophy points
1,393
Activity points
18,441
I have to side with AudioGuru on this one. Are we talking about DC resistance or AC impedance?

It is correct that for the AC analysis the source resistor would be completely shunted away by the infinite capacitor in parallel with it....in which instance the problem would require to find the input impedance, not the input resistance.

In other words, the problem statement is confusing. It should have stated to find the input and output impedance of the circuit, even though that impedance for all frequencies other than DC, the magnitude of Zeq = Req, and the angle would approach zero.


I know that I am sounding pedantic, but a prof actually screwed me, over 46 year ago, precisely with a very similar problem statement.
 

Audioguru

Advanced Member level 5
Joined
Jan 19, 2008
Messages
9,327
Helped
2,149
Reputation
4,292
Reaction score
1,975
Trophy points
1,393
Location
Toronto area of Canada
Activity points
58,709
I was wrong because I did not see the source capacitor, which might have been added later to increase the gain.
 

BigBoss

Advanced Member level 5
Joined
Nov 17, 2001
Messages
5,182
Helped
1,537
Reputation
3,074
Reaction score
1,390
Trophy points
1,393
Location
Turkey
Activity points
31,154
There is no AC but DC Feedback.Analysis seems correct for AC only..
 

albbg

Advanced Member level 4
Joined
Nov 7, 2009
Messages
1,256
Helped
434
Reputation
868
Reaction score
389
Trophy points
1,363
Location
Italy
Activity points
9,491
I have to side with AudioGuru on this one. Are we talking about DC resistance or AC impedance?

It is correct that for the AC analysis the source resistor would be completely shunted away by the infinite capacitor in parallel with it....in which instance the problem would require to find the input impedance, not the input resistance.

In other words, the problem statement is confusing. It should have stated to find the input and output impedance of the circuit, even though that impedance for all frequencies other than DC, the magnitude of Zeq = Req, and the angle would approach zero.


I know that I am sounding pedantic, but a prof actually screwed me, over 46 year ago, precisely with a very similar problem statement.

The OP talk about "small signal parameter", then DC biasing is not involved in the analysis.
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top