# Dislike my Lab class: semicon. circuits

• tim_lou

#### tim_lou

I'm currently enrolled in a lab class about circuits. we are doing diodes, transistors, op-amps these days.

the thing is, I've never taken any circuit analysis before, but the prof. expects us to be able to understand op-amp, diodes, transistors... and be able to calculate quantities for complicated circuits involving these things.

My question is what equation governs ideal transistor and op-amp?

for instance, for resistor, V=IR.

what about transistors and op-amps? I have searched for many books in the library (they are either too complicated or too simple to include diodes) and struggled with my book (the book is like a layman introduction to circuits... I don't like that style... the math is sketchy and derivations are usually omitted)

I know for ideal transistor, current through emitter = current through collector...is that correct? what about voltage?

and for op-amp... I really don't know.
so for op-amp, what is the governing equations/relations regarding
1. -input
2. +input
3. output
in terms of voltage and current?

I just need to know the fundamental equations about these things... It will be appreciated if anyone can help me... thanks for your reading.

Check out a copy of Horowitz and Hill, Art of Electronics for a straightforward no-nonsense presentation of all things electronic.

I know for ideal transistor, current through emitter = current through collector...is that correct? what about voltage?

You can say that since the base current is very small compared to emitter current. The voltage across base-emitter and base-collector is the same principle as a diode when forward biased and reversed biased. Remember that a diode consist of n-type and p-type material (NP), and a bjt transistor consist of either NPN or PNP.
and for op-amp... I really don't know.
so for op-amp, what is the governing equations/relations regarding
1. -input
2. +input
3. output
in terms of voltage and current?
Its too broad of a question. What kind of op-amp?

Perhaps you need to step back and enroll into a circuit analysis class that teaches op amps, bjts, and diodes before you take this class.

Yeah man, that class would be crazy without the necessary foundation. We used Microelectronic Circuits (by Sedra and Smith) in my course, I really liked this book.

I'm currently enrolled in a lab class about circuits. we are doing diodes, transistors, op-amps these days.

the thing is, I've never taken any circuit analysis before,

my question is how do you find yourself in an electronic circuits lab and you've never had a class with KCL, KVL, loop-current, or node-voltage, etc?

but the prof. expects us to be able to understand op-amp, diodes, transistors... and be able to calculate quantities for complicated circuits involving these things.

My question is what equation governs ideal transistor and op-amp?

for instance, for resistor, V=IR.

strictly speaking, i think it's the Ebers-Moll equation for the ideal BJT and op-amps are s'pose to be ideal voltage-controlled voltage sources with low output impedance and high input impedance. but don't deal with the Ebers-Moll eq. instead...

what about transistors and op-amps? I have searched for many books in the library (they are either too complicated or too simple to include diodes) and struggled with my book (the book is like a layman introduction to circuits... I don't like that style... the math is sketchy and derivations are usually omitted)

I know for ideal transistor, current through emitter = current through collector...is that correct? what about voltage?

... deal with the transistor as a current-controlled current source with some output impedance and a pretty low input impedance.

i don't know what the pedagogy is that you're in, but you should have had the fundamentals of circuit analysis before getting to the lab course you're in.

thx for the replies

Its too broad of a question. What kind of op-amp?

sorry about that. I meant op-amp 741. the book presents a bunch of usages of this thing, including inverting amplifier, non-inverting amplifier, differentiator and integrator... the book never says anything about principle equation... for instance, in the derivation of gain in non-inverting amplifier, the book says that the negative input and positive are at the same voltage (for op-amp 741). I don't know if that is true or not... is it true in general? or??

my question is how do you find yourself in an electronic circuits lab and you've never had a class with KCL, KVL, loop-current, or node-voltage, etc?

the class is designed for physics majors... it is just a part of the standard computer based lab for physics majors. the prereq. is just some freshman "modern physics" class... anyway.

rbj said:
deal with the transistor as a current-controlled current source with some output impedance and a pretty low input impedance.

so, I think I know what you are talking about... the T approximation or something like that? I think I got that one.

perhaps you can tell me what the op-amp is equivalent to in a circuit? (approximately?)

thx for the replies

sorry about that. I meant op-amp 741. the book presents a bunch of usages of this thing, including inverting amplifier, non-inverting amplifier, differentiator and integrator... the book never says anything about principle equation... for instance, in the derivation of gain in non-inverting amplifier, the book says that the negative input and positive are at the same voltage (for op-amp 741). I don't know if that is true or not... is it true in general? or??
It is the bunch of uses for the op-amp that give rise to the numerous equations. There isn't any principle equation to relate those aspects that you've requested. It just changes from application to application. As for the gain for a non inverting amp, I'm afraid that it isn't the same principle for the other op-amp configurations.

The circuit to the right shows a non-inverting op amp circuit. In this circuit, the input signal is effectively used as the reference voltage at the "+" input to the differential amplifier, while the "-" input is indirectly referenced to ground. In order to keep the two input voltages to the amplifier the same, the amplifier must set Vout to whatever voltage is required to make the feedback voltage to the "-" input match the input voltage to the "+" input.
http://www.play-hookey.com/analog/non-inverting_amplifier.html [Broken]

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the class is designed for physics majors... it is just a part of the standard computer based lab for physics majors. the prereq. is just some freshman "modern physics" class... anyway.

so, I think I know what you are talking about... the T approximation or something like that? I think I got that one.

perhaps you can tell me what the op-amp is equivalent to in a circuit? (approximately?)

IMHO, I find it rather surprising that they would give a circuits lab without a proper circuits analysis course to complement it. Regardless of your major, you need to have the theory down before you can understand the lab. This is just bad planning on their part.
http://content.answers.com/main/content/wp/en-commons/thumb/1/12/450px-Opamptransistorlevelcoloredlabeled.png [Broken] is the schematic of a 741. So it equivalent to transistors, resistors, and capacitors in special configurations to perform the desired functions.

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It is the bunch of uses for the op-amp that give rise to the numerous equations. There isn't any principle equation to relate those aspects that you've requested. It just changes from application to application. As for the gain for a non inverting amp, I'm afraid that it isn't the same principle for the other op-amp configurations.

really? looks like I'll have to memorize the results for inverting, non-inverting, instrumentation amplifier... and all the other good stuffs. the 741 does look like a complicated piece of device... Just hope my prof doesn't ask me to work out a circuit with a new configuration of 741s...

There is no need to memorize everything. Invest some time to learn KCL and KVL. They will help you a lot. You can also apply these concepts to transistor circuits.