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Feedback Amplifier Design Project @ 5th Semester

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TheExtremist

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Hey ;


I am a student of Yeditepe University in Turkey at 5th semester.Today,our EE331 (Electronics I) course instructor gave us an feedback amplifier design project with respect to given specifications below ;

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Input Impedance = 50k

Input Sensitivity = 2.5mV rms at 1Khz

Rated Output = 2V peak at rated input sensitivity across a load of 10k

Bandwidth = 20hz to 20khz (-1dB)

T.H.D. =Less than 0.1% from 20hz to 20khz at rated output


Hum And Noise = -65 dB at rated output at 1 KHz

Power Supply Requirements = Single


Maximum Transistors Per Project = 2

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I am so confused that i do not know how to start calculations etc. Should i realize this circuit MOS or BJT ? How can i determine the feedback topology, amplifier configuration (CE,CC.. etc.).So i need your help,recommendations to create my own feedback amplifier.
 
The idea is to look at the characteristics of the various amp configurations (CE,CC, etc.) and, from that, determine which configuration best meets the amplifier requirements. For example a CE amp can have a gain >1, but has a relative low input impedance and high output impedance. A CC amp (emitter follower) has a gain of<1, but has a high input and output impedance.

Since the amp has low distortion, you will need negative feedback in the amp. This could be from output to input, or localized such as an emitter resistor in a CE amp.

You can build the amp with either MOSFETs or BJTs. MOSFETs have a high input impedance in all configurations, but are a little more difficult to bias because of the large variation in the gate threshold voltage from unit-to-unit. They also tend to have more distortion because of the non-linear gate voltage to source current relationship, which requires more feedback to meet a given distortion requirement.

Do you have a program such as LTSpice to simulate the circuit? That's a great way to develop and verify your design.
 
Thanks for your quick reply :)


I always use PSPICE version 16.2 to simulate circuit characteristics.I think I should build my circuit by using BJT's,because in lessons we mostly study on BJT nowadays and also as you said in MOS it is hard to set the gate voltage.

My input impedance is 50k , i remember that there is MegOhms at MOSFET's.So , using BJT instead of MOS is more reasonable for me, isn't it?
 
Can i calculate the gain by;

İnput sensitivity = 2,5 mV RMS

2,5 mV = 3,54 mV Peak. Then ;


A= 2V/ 3,54mV = 577


Is this calculation for gain correct?
 
A= 2V/ 3,54mV = 577

Is this calculation for gain correct?

Not exactly. To convert RMS to its peak value as you have displayed, is to raise it by a factor of 2 X sin 45 or 2.5mv X 2 sin 45 = 3.5355mv. Then Av = 2v/3.5355mv = 565.7. But that is the gain objective after the feedback is applied. So the open-loop gain must be greater. Here is a hint; 3db below an Av of 800 is an Av equal to 566.

My input impedance is 50k , i remember that there is MegOhms at MOSFET's.So , using BJT instead of MOS is more reasonable for me, isn't it?

That is the hard nut to crack. Both the BJT and FET can be used for this given the Zin criteria, but requires some out of the box thinking. In the end, that would be your call as a budding engineer.

One bit of advice, don't discard any option without some investigation first; class notes, textbooks and other sources. After all, if called upon, you may have to defend your choices.
 
I told my questions to my instructor and then she gave me some hints;

1)In order to set the input resistance easily she suggested me to use MOS instead of BJT.Because in BJT's small signal modell the input resistance consists of R1//R2//R∏ (where R1 R2 are Base Voltage Divider resistances and R∏ is the resistance occurs from fabrication process).She told me that you can easily set the input resistance by only choosing Voltage Divider resistances in MOS.

2)How can i calculate the gain? She asked this question ;

You can calculate it by dividing Output/İnput.In the project that gave you, i mentioned input , by saying "input senstivity". So our input sensitivity is our input (2,5mV RMS).By doing appropriate calculations (RMS-Peak) you can calculate your gain.


3)How Hum or THD effect our circuit design?

She said that do not think about them i gave that specifications just for extra information.The important thing is your feedback configuration.




In this project we can see that that is voltage amplifier.But how can i decide its feedback configuration shunt-voltage or series-voltage?
 
Hi Extremist

I told my questions to my instructor and then she gave me some hints;

1)In order to set the input resistance easily she suggested me to use MOS instead of BJT.Because in BJT's small signal modell the input resistance consists of R1//R2//R∏ (where R1 R2 are Base Voltage Divider resistances and R∏ is the resistance occurs from fabrication process).She told me that you can easily set the input resistance by only choosing Voltage Divider resistances in MOS.

Go with which device you are most comfortable with for sure for this assignment or which will be the greater challenge for you. Learning and taking away added knowledge is the objective. I thought I would also take it on as a refresher so I chose using BJT's.

Now the complete equation for a BJT's input impedence, not to contradict your professor, is as follows:
Zin = Ra // Rb // B(Re + 26mv/Rei) where Ra & Rb are the base resistors, as you noted, Re is the non-bypassed emitter resistance of the emitter circuit, the 26mv is a physical constant of the emitter/base diode at 1ma (silicon or germanium), which is linearly scaleable and also referred to as "re", Rei is the emitter current and B is the device beta. This yields a very close approximation below about 10Mhz, above which one must resort to using the "h" parameters from the datasheet of the device and all those laborious equations. Use of a spreadsheet helped me converge on the solution, which resulted a Zin of 50,012 ohms using E96 resistors (1%).

The above described configuration is what I meant by thinking out of the box. I'll post the basic schematic without values below in case I haven't been clear.

2)How can i calculate the gain? She asked this question ;

You can calculate it by dividing Output/İnput.In the project that gave you, i mentioned input , by saying "input senstivity". So our input sensitivity is our input (2,5mV RMS).By doing appropriate calculations (RMS-Peak) you can calculate your gain.

Given Av = Vout / Vin, both terms must be either RMS, Peak or Peak-Peak. Since the input was given in RMS and the output in Peak voltage, it can be written as either Av = 2V / 2.5mv*2sin 45 or Av = 2V*sin 45 / 2.5mv, which both yield the same solution of Av=565.7.

3)How Hum or THD effect our circuit design?

She said that do not think about them i gave that specifications just for extra information.The important thing is your feedback configuration.

In this project we can see that that is voltage amplifier.But how can i decide its feedback configuration shunt-voltage or series-voltage?

Consider the purpose of feedback in general. If one wishes a signal to be increased it must be in phase, which is a prime tenet of Birkoff's(sp?) Law of oscillation; regenerative feedback of the proper amplitude. On the other hand, to limit a signal's amplitude, the feedback must be degenerative in nature; of the proper amplitude but 180 deg out of phase to obtain the desired output level.

If you know what the input and output levels are at the second stage, the ratio gives you the amount of feedback required. Since the overall gain is to be 565.7, it is obvious that your overall gain before feedback must be somewhat greater. How about an Av of 800, which is precisely 3db higher meaning a required feedback of that degree.

So after you design your amplifier and the gain is known, look at it and find the best location for the feedback that will not interfere with the I/O impedances, that does not impact DC bias levels and is the correct phase.

I hope this helps. Good Luck.
 

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