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I'm studying the four kinds of amplifiers implemented through feedback in op amp circuits. When trying to relate all my previous knowledge with this, I'm getting a bit confused.

Can any BJT, MosFET, or JFET transistor be an amplifier of any kind? Are there advantages of BJT being a specific kind of amplifier?

Now, considering a basic op amp (voltage difference amplifier), can it be used as any of the four amplifiers (current, voltage, transcondutance, transresistance), how?

Can be a op amp circuit (inversor, non-inversor, summer, etc.) any of the four amplifiers (current, voltage transcondutance, transresistance)?

If that's possible, how would they be implemented?

What i want to know the most is like a yes/no question.Here it is : Can the four amplifiers configurations be made interchangeably from any (one unit ) electronic device ( bjt,jfet,mosfet) or integrated-circuit ( basic op amp and op amp circuits )?

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  • \$\begingroup\$ It's hard to tell what you're asking. Are you asking how to use an op-amp to build different types of amplifiers? Or are you asking how to use discrete components to build different types of amplifiers, and how those are put together to make an op-amp? \$\endgroup\$ – The Photon Dec 12 '12 at 22:01
  • \$\begingroup\$ Sorry for not expressing myself precisely.I'm asking if its possible to build any kind of amplifier using discrete components.And if it is possible would it be simple? Next, i was asking if one simple and basic op amp (voltage difference amplifier ) can be used as any kind of amplifier ( not taking voltage difference as input for example ).And if is possible, i was wondering how. \$\endgroup\$ – Luiz Costa Dec 12 '12 at 22:17
  • \$\begingroup\$ Answering all of that could be pretty involved. It might make sense to break this up into 2 or 3 questions. \$\endgroup\$ – The Photon Dec 12 '12 at 22:41
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    \$\begingroup\$ You seem to be asking multiple questions. You would do better if you picked one. \$\endgroup\$ – Brian Carlton Dec 12 '12 at 23:01
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    \$\begingroup\$ If that's the main question, please edit the question. Consider splitting off the additional questions into separate question items. \$\endgroup\$ – Kaz Dec 17 '12 at 6:00
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Can any BJT, MosFET, or JFET transistor be an amplifier of any kind? Vaguely, yes. However, certain transistors are better at amplifying different types of signals in different ways.

Are there advantages of BJT being a specific kind of amplifier? BJT's have a wide linear region compared to MOSFETs, however, JFETs typically have an even wider linear region. Unfortunately, JFETs are typically more expensive compared to a similar BJT. Also JFETs are more difficult to use (negative bias at the gate AND really easy to let the magic smoke out). They also don't typically come in higher currents and voltages. So, BJTs are better at mid and high power analog circuits (even though they have their disadvantages too: lower input impedance, less linear than JFETs, REALLY needs temperature compensation for high power uses, and more..)

Now, considering a basic op amp (voltage difference amplifier), can it be used as any of the four amplifiers (current, voltage, transcondutance, transresistance), how? Yes, a standard op amp can be made to amplify these four ways. To accomplish this, you don't. You buy an opamp made for the type of amplification you are doing (e.g. a transconductance op amp) because they will work significantly better than modifying a basic op amp. If it's all you have.. google " 'type of amp' op amp circuit" (e.g. "transconductance op amp circuit") and click images. Sorry, I am lazy - this would take forever.

Can be a op amp circuit (inversor, non-inversor, summer, etc.) any of the four amplifiers (current, voltage transcondutance, transresistance)? I'm not sure how this is different from the previous question. One MAJOR disadvantage to op amps is that they usually can only go -15V to +15V and < 100mA max. You can buy power op amps, but they usually aren't very linear and are expensive. At this point you use discrete transistors to bring the voltage up. EX: a 100W stereo typically can swing from (approx) +30 to -30V at 2A. To do this, almost every decent stereo uses discrete transistors in the final stage.

If that's possible, how would they be implemented? same as above.

What i want to know the most is like a yes/no question.Here it is : Can the four amplifiers configurations be made interchangeably from any (one unit ) electronic device ( bjt,jfet,mosfet) or integrated-circuit ( basic op amp and op amp circuits )? With circuits, you can ghetto-rig just about anything with a handful of transistors, resistors, capacitors, inductors and a power supply. If you have a handful of op amps as well, you can make the same circuits but smaller and usually a little better.

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Can the four amplifiers configurations be made interchangeably from any one electronic device?

As far as I know, the answer is yes. The differences between each of the amplifier types (Voltage, Current, Transresistance, and Transconductance) is more related to the structuring of the circuit and feedback network as opposed to which components are used. However, different components will have difference benefits such as linear region, input resistance, and speed, as Danny Kmack mentioned previously.

For example, using an op amp one can obtain a voltage amplifier by using a non-inverting or inverting configuration which will provide an output voltage from an inputted one. Alternatively, in the inverting configuration if a current is inputted to the inverting terminal with a resistor providing a negative feedback, you can get a voltage output from a current input with the relationship V = -I*R (where R is the resistance providing negative feedback). If you wanted to obtain a current output from a voltage input, you can simply place your load resistor in the negative feedback loop to get an output current with the relationship I = -V/R (where R is the input resistance) [Tried to post image but I can't due to being a new user]

Then I think you can use the same configuration with an input current to get a current amplifier.

For transistors its a similar case, but I believe the type of amplifier mostly depends on your feedback network: series-series for a transconductance amplifier, series-shunt for a voltage amplifier, shunt-series for a current amplifier, and shunt-shunt for a transresistance amplifier. (Schematics for all of these are not posted to avoid making this post excessively long)

As a closing note, Texas Instruments has a good document for operational amplifier applications which covers a lot of different circuit configurations for different applications. A copy of this document can be found here:

http://www.ti.com/lit/an/sboa092a/sboa092a.pdf

Please correct me on any of this if I've provided incorrect information.

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