What properties must have OPA2 in annex in order to work the circuit correctly?
We suppose UREF 0 to 1 V.
Another puzzle? Maybe related to the previous question?
If so, maybe it should be about the implementation of the OPA2 inputs... or outputs...
For example, OPA1's output should source current while OPA2's output should sink current. But conventional op-amps can do both...
Both stages are the same - improved by an op-amp transistor current sink/source...
Hi Cyril
it is as delicate situation as in the first case.
Just imagine that Uref is 0,1 V - what about inputs voltage of OPA2 - near Ucc+!
And what about output capability of OPA2! Will be it able to control a transistor?
With "741" OPA type it will not work! But solution exists!
Josef
The situation with an ordinary OPA - see Annex.
Now apply "green data".
I did something similar for driving an IR laser diode at up to 2.5 amps, ended up using a Darlington output transistor.
Modeling this circuit for 0 to 3.5 volts in to give 0 to 3.5 amps out (again thinking about use for a LD driver) with a +5V supply rail, a standard PNP power transistor needed about -78mA output for the base, which is a bit hard for many op-amps! Changing to a TIP147 took that down to 0.6mA which is better.
Yes Cyril, we must use RRIO type as OPA2 (rail-to-rail input/output)
Josef
Even simpler solution would be not to come close to the power supply rails:)
Not exactly. The output is not needed to be RR. Only the opamp input common mode voltage range is important, in OPA1 it should include ground, in OPA2 supply rail.
Hi Cyril
"Even simpler solution would be not to come close to the power supply rails"
But it is not elegant solution - and output voltage range will be reduced.
Because we would have used input voltage about 3 V (or greater).
Josef
Hi Dobromir
In our case OPA2 must be RRIO to be able to control current of an output transistor for small input voltage. Another possibilities see for example pgs 211-212 in
https://www.researchgate.net/publication/281480600_Theory_of_electronic_circuits
Josef
Book Theory of electronic circuits
Nice to see a mention of vacuum tubes in the "Theory of electronic circuits" text book :)
Hi Josef,
By two resistors, the opamp output can be shifted anywhere in the supply range.
So, the opamp output is not needed to be RR.
Hi Susan
Not bad to show that the tubes does the same thing as for transistors.
After all tubes belong to one of the basic principles, as well as FET and BJT.
Josef
I still use tubes in some of my audio projects, currently building a RIAA preamp using C3gS tubes and transformer coupling :)
Downvote for vacuum tubes - it really is "brave" - and funny :-)
Why should we not know the tubes?
Whether we like it or not, it is still existing principle of amplifying elements that you need to know.
We should be able to "read" historical circuits.
Still they bring a lot of good ideas.
An example of a powerful "historical idea" originating from tube times - the cathode (emitter, source) degeneration.
I like vacuum tubes as (in most cases) one can actually see the basic elements of the active device and it is (relatively) easy to visualize the operation.
This is unlike semiconductors which are microscopic in operation and normally tucked away in a blob of something - with an arcane set of letters and numbers printed on the outside which ironically can cause great excitement or a yawn of indifference.
Also vacuum tubes operate on real electron flow, whereas most other circuit paths are electron wave in operation (the electrons themselves moving at very slow speeds (down to cm/sec which is finger waving velocities!)).
And there are still a few circuits where a vacuum tube can do things that solid state cannot. So I see vacuum tube electronics as a niche specialty, rather than just something from history.
Hi Cyril and what about cascode or current source in "emitter" - see annex.
Josef
A great idea where a voltage source (tube, emitter or source follower) "helps" a current source (common anode, collector or drain stage). The same situation exists in the differential stage above when we adjust the current produced by the common emitter current source and the two emitter followers (connected in parallel) react by changing their collector currents and emitter voltages...
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