Sam Goldwasser (sam@stdavids.picker.com) wrote: : In article <4qtau3$9l@jagalchi.cc.pusan.ac.kr> ysaum@hyowon.cc.pusan.ac.kr (Youngsub Aum) writes: : > Thank you 4 read this : > I want 2 have Amplifier which have 100 gain. : > Just use NPN transistor. : > Theoritical design is not a Perfact Answer. : > What make it deficult? : > Some help...... PLEASE Which gain -- Av, Ai, or Ap?? I assume we're talking about Av. : How close to 100 do you want it? The nice thing about op amps is that with : very high open loop gain, controlling the closed loop behavior is for many : applications very accurate and the theory works. : With a single NPN of high enough Hfe, you can get 100 gain easily - you will : need to use an emitter resistor as feedback to stabilize the collector : current vs. input voltage. The gain will be roughly the ratio of the collector : to emitter resistances. However, since Hfe is not infinite and can vary : quite a lot from transistor to transistor - or with temperature, etc. - this : will not be nearly as constant as an op amp. : o : | : / : \ : / Rc : \ : | : +-----||-----o Output : | : |/ C : Input o----------| NPN : |\ E : | : / : \ Re : / : \ : _|_ : - : (Biasing not shown). : Vout (small signal) ~= Vin * Rc/Re. The Re results in the transistor : operating as an emitter follower which means that Vin appears on Re. : Since nearly the same current (actually, reduced by Hfe/(Hfe+1)) flows : through Rc, the voltage across Rc will be Rc/Re times the voltage on Re. The voltage gain is high only as long as the Rl on the output is >> Rc. : With multiple transistors, however, you can design a very stable high gain : amplifier. The usual preamp stage has two transistors in a shunt series connection. They can have nice flat gain with wide bandwidth. The negative feedback works wonders. See H&H's Art of Electronics for more info, as he states below. : See any introductory book on transistor circuit design or Horowitz and Hill.. : --- sam -- #======P=G=P==k=e=y==a=v=a=i=l=a=b=l=e==u=p=o=n==r=e=q=u=e=s=t======# | John Lundgren - Elec Tech - Info Tech Svcs. | jlundgre@ | | Rancho Santiago Community College District | deltanet.com | | 17th St at Bristol \ Santa Ana, CA 92706 | "http://www.rancho"| | My opinions are my own, and not my employer's. | .cc.ca.us | | Most FAQs are available through Thomas Fine's WWW FAQ archive: | |"http://www.cis.ohio-state.edu:80/hypertext/faq/usenet/FAQ-List.html"| | "You can flame your brains out -- it won't take long." |
Date: 6 Jul 1996 17:18:22 GMT
Original Subject: Re: Q: 100gain AMP without OP-AMP ??