Tradeoff between gain and impedance - Amplifier

[bugmenot]

It seems to me there is a fundamental tradeoff between the gain of a common emitter amplifier and its Zin:Zout ratio. If you make the Rc high, you increase gain, but also raise Zout. If you make the Ze high, you raise Zin, but you also lower the gain. This seems to me a fundamental tradeoff, solvable only using emitter-follower's to amplify the current. Am I correct?

 

[MrAl]

Hi there,

Many devices are like this, you accept trade offs between various operating
modes.
When you reduce the Re or Ze you also reduce the input Z too.
If you increase Rc you increase the output Z.

The transistor, as with many devices, cant do everything...it's got limitations
too.

 

[Externet]

:Welcome here Al.
Miguel

 

[MrAl]

Oh hi there Miguel! Thanks, and nice to see you here too.
I just found this site a week or two ago.

Take care for now...

 

[audioguru]

A single transistor amplifier circuit with high gain has severe distortion when its output level is fairly high. You should use an opamp instead.

Here is a simulation of a single transistor without negative feedback. its voltage gain is about 190 and its distortion is 40%.

The same amplifier has negative feedback added so that its voltage gain is reduced to 10. Its distortion is reduced to 3%.

 

[bugmenot]

audioguru - It seems to me that the distortion comes because the Zout of the source is 4k, and the Zin at 1kHz is only around 50Ω (re of the transistor, plus a drop from the capacitor). But, if the signal source was Zout 1k (easily achievable with an emitter follower), and you add a 55Ω resistor in parallel with the capacitor, the distortion should go away, no? Can you simulate that also?

BTW - great simulation - what software did you use for it?

 

[audioguru]

My simulation and schematic are made with SwCAD III software that is free from Linear Technology.
In my sim the source impedance is zero, that is why the voltage gain of the transistor is so high. The input impedance of the transistor circuit is 6.5k ohms on the datasheet of the 2N3904 but is calculated as 12k ohms.
Distortion is reduced if the collector has a constant current source (used in opamps) instead of a resistor that provides less and less current as the output voltage tries to rise. Bootstrapping also reduces distortion but the bootstrapping capacitor needs to be driven from a low impedance emitter-follower.

 

[Roff]

audioguru - It seems to me that the distortion comes because the Zout of the source is 4k, and the Zin at 1kHz is only around 50Ω (re of the transistor, plus a drop from the capacitor). But, if the signal source was Zout 1k (easily achievable with an emitter follower), and you add a 55Ω resistor in parallel with the capacitor, the distortion should go away, no? Can you simulate that also?

BTW - great simulation - what software did you use for it?

Zout of the source is zero. 4k is the frequency. The input impedance is beta times re, in parallel with the bias network. The nonlinearity comes from the fact that gain≈Rc/re, and re is dependent on emitter current. Re≈.026/Ie , and Ie varies with instantaneous signal amplitude.

 

[Analog]

It seems to me there is a fundamental tradeoff between the gain of a common emitter amplifier and its Zin:Zout ratio. If you make the Rc high, you increase gain, but also raise Zout. If you make the Ze high, you raise Zin, but you also lower the gain. This seems to me a fundamental tradeoff, solvable only using emitter-follower's to amplify the current. Am I correct?

Your are correct. But what you are proposing are rudimentary amplifiers, useful only in electronics classes and learning. A real amplifier uses active loads, with a differential pair up front and negative feedback. Open loop gains of these types of amps may be over 100,000. Closed loop gain gives you low distortion, and a proper feedback network kills any DC gain so that you don't fry the speakers.

 

[audioguru]

Hee, hee,
Simple "theory" amplifiers are used in school. They work very poorly.
Real amplifiers are more complex and have current sources insted of resistors and have plenty of negative feedback.

 

[bugmenot]

Hee, hee,
Simple "theory" amplifiers are used in school. They work very poorly.
Real amplifiers are more complex and have current sources insted of resistors and have plenty of negative feedback.

Right, and even "real amplifiers" that I may build work poorly, compared to what I could buy for around the same money. The point is, I'm trying to teach myself basic EE and circuits - not build a hifi or ipod - and the best way to do this is to start from the ground up (no pun intended).

 

[audioguru]

I got a portable AM radio for my birthday about 53 years ago. Its circuit was horrible but i thought the radio was wonderful since I was just a kid.
Circuit design gives very much better performance today. But unfortunately schools teach only about the horrible old circuits.

The web has much more modern parts, tutorials and explanations about modern circuits.

 

[Wond3rboy]

Hee, hee,
Simple "theory" amplifiers are used in school. They work very poorly.
Real amplifiers are more complex and have current sources insted of resistors and have plenty of negative feedback.

i am learning fomr the art of electronics do you know any site that helps you in professional or worthwhile amp designing?

 

[BeeBop]

i am learning fomr the art of electronics do you know any site that helps you in professional or worthwhile amp designing?

try this one:
**broken link removed**

 

[audioguru]

Douglas Self has some good theory about modern amplifier design:
**broken link removed**