# a BJT Op

Discussion in 'Homework Help' started by NCH, Feb 28, 2013.

1. ### NCH Thread Starter New Member

Feb 28, 2013
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Hello there! This is my first post here so i hope i get this about right.
My english isn't really perfect but ill try to keep it at a normal level
I've gotten an asignment to simulate a OP amp with BJT and resistors!

I've already figured out this isn't as easy as it seems thru several attempts at it. Really the biggest issue i have is that i don't really know the real analysis of the circuit at a basic level. I'm looking for someone to give a explanation of the different parts that i've figured out the OP consists of.
Probably the most important part is the current-mirror and the input stage of the OP.

I can't really figure out how to choose my hFe for each of the pnp/npn transistors. Any help is greatly appreciated! A have a basic level of electronics so you know at what level i could use somekind of help.
My electronic teacher said that some pairs of transistors shouldn't have the same hFe but that didn't really help me much with this i feel..
Where should i begin with this?

Here's an example of the circuit!
http://imageshack.us/photo/my-images/94/skrmavbild20130220kl150.png/

2. ### ScottWang Moderator

Aug 23, 2012
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992
Where did you get the idea of that circuit?
Where is the Totem Pole of output, as uA741, LM358, LM324, Op07, etc...
No kidding, you want to build a new type of output?
You can check the output of those Op Amp.

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
I'm guessing the OP's task is not to re-invent a commercial op-amp. Rather the task seems to be to gain a simple understanding of the essential parts of an op-amp. In principle the op-amp is a differential input signal amplifying device having substantial gain-bandwidth and a single ended output. The attached schematic from NCH essentially fits that concept - using discrete components. The inclusion of a totem pole output isn't an absolute necessity.

The OP hasn't really made it clear whether the requirement is to produce a physical working circuit or a simulator based equivalent.

From NCH's perspective I'm surprised that this task has been given to someone with only "a basic level of electronics". In any event, there is more than sufficient information on BJT based differential amplifiers [input stage] and current mirrors on the web. Google is probably your friend in this case.

For example, there is a discussion of current mirrors here ...

Last edited: Mar 1, 2013
4. ### NCH Thread Starter New Member

Feb 28, 2013
12
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well the task itself is to be adle to use the circuit for simulation but if this circuit would even be enough i'm not sure of, its only an example given to us be our teacher. The simulation does require us to use it to figure out different things about the OP. Such as bandwidth,risetime,gain,offset,cmrr railtorail etc..

Would you say this circuit is enough to simulate this?

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
Yes it will be fine - but you will have to adjust values according to transistor characteristics and this would require both a careful DC & AC analysis to design and then set up [with some tweaking] the correct DC operating conditions and expected gain performance.

In other words you need a design specification as a starting point.

Feb 28, 2013
12
0