Constant current source composed of field effect tube

fishsheep1919

Constant current source composed of field effect tube [Copy link]

 

Is there any teacher who can help analyze this circuit? Is this circuit a constant current source determined by the voltage of the voltage regulator tube and the resistance value of R1? Do both field effect tubes play the role of amplifying current?

maychang

"Is this circuit a constant current source determined by the voltage of the Zener diode and the resistance value of R1?"

yes.

maychang

"Do both field-effect transistors amplify current?"

There are no two field effect transistors, only one.

fishsheep1919

maychang posted on 2021-2-18 10:41 "Is this circuit a constant current source determined by the voltage of the Zener tube and the resistance value of R1? " Yes.

So what is the specific working principle and practicality?

maychang

This circuit may be wrong. uA741 cannot work with a +/- 30V dual power supply. This power supply voltage exceeds the allowable supply voltage range of uA741.

maychang

fishsheep1919 posted on 2021-2-18 10:43 So what is the specific working principle and practicality?

"Then what is the specific working principle and practicality?"

Working principle: Two amplifiers in cascade are still an amplifier, just like two resistors in series are still a resistor.

Q1Q2 and U3 are still an amplifier after cascading. When working in a linear state, the non-inverting input terminal and the inverting input terminal are required to be "virtually shorted", that is, the voltage across D1 is equal to the voltage across R1. Therefore, the current passing through RL is equal to the voltage across D1 divided by the resistor R1, which has nothing to do with RL, so it is a constant current source.

maychang

fishsheep1919 posted on 2021-2-18 10:43 So what is the specific working principle and practicality?

"Then what is the specific working principle and practicality?"

The practicality is relatively poor, as can be seen from the power supply voltage of plus or minus 30V: it is not a serious design.

The post does not specify how much constant current is required on the load, nor how high the voltage is required on the load. It is estimated that it is not necessary to use two tubes, and only one MOS tube is enough.

fishsheep1919

maychang posted on 2021-2-18 10:57 "Then what is the specific working principle and practicality?" The practicality is relatively poor, as can be seen from the power supply voltage of plus or minus 30V: it is not a serious design. ...

I just checked, it seems to be ±18V. I tried to use a MOS tube, but then there is no voltage on the load resistor. . .

fishsheep1919

maychang posted on 2021-2-18 10:57 "Then what is the specific working principle and practicality?" The practicality is relatively poor, as can be seen from the power supply voltage of plus or minus 30V: it is not a serious design. ...

Constant or nA or uA current

maychang

fishsheep1919 posted on 2021-2-18 11:10 Is it constant or nA or uA level current

If you want to keep a constant nA or uA current, such a circuit is basically unusable, because the leakage current between the drain and source of a low-power MOS tube is at the uA level.

maychang

fishsheep1919 posted on 2021-2-18 11:10 Is it constant or nA or uA level current

There was a similar constant current source post a few days ago, maybe it was your fellow teacher, who also required a stable nA or uA level current. In the reply to that post, I told him: Use a dual power supply Howland circuit.

maychang

fishsheep1919 posted on 2021-2-18 11:10 Is it constant or nA or uA level current

Why constant current source simulation does not produce constant current

Is this circuit a constant current source?

fishsheep1919

maychang posted on 2021-2-18 11:31 There was a similar constant current source post a few days ago. It may be your fellow apprentice, who also requires stable nA or uA current. In the reply to that post, I told...

Thanks, that's me. In the second post I designed a simple Howland circuit, but I don't know how to add the FET.

chunyang

First of all, why do we have to add field effect transistors? Is it a homework assignment?

maychang

fishsheep1919 posted on 2021-2-18 13:11 Thank you, that's me. In the second post, I designed a simple Howland circuit, but I don't know how to add the field effect tube

Since the Howland circuit is used and the output current is only at the uA level, there is no need to use a field effect tube, and the op amp can output directly. Any type of op amp can output 1mA level current.

maychang

fishsheep1919 posted on 2021-2-18 13:11 Thank you, that's me. In the second post, I designed a simple Howland circuit, but I don't know how to add the field effect tube

However, the output requires uA-level current and stable constant current, so the op amp needs to use a precision op amp. Otherwise, the temperature drift of the op amp will be enough to make your constant current source lose its way. Each resistor also needs to use a low temperature coefficient resistor.

fishsheep1919

chunyang posted on 2021-2-18 13:47 First of all, why do you have to add field effect tubes? Is it specified in the homework?

It's like this. I'm a student and this is my first time to come into contact with this kind of thing. Then the teacher asked me to make a constant current source to power the accelerometer. Our teacher didn't do much research in this area. I heard him mean to design a constant current source based on field effect tube, like the Howland constant current source mentioned by teacher Maychang. I'm afraid that if the teacher has never come into contact with it, and if the accuracy requirements are improved or changes are required, or boards are drawn, or wiring is required, I won't be able to figure it out. But if the accuracy design is simpler than the field effect tube constant current source, I will discuss it with the teacher and it's not a bad idea to try another way.

fishsheep1919

maychang published on 2021-2-18 14:14 However, the output requires uA-level current and constant current stability, so the op amp needs to use a precision op amp. Otherwise, the temperature drift of the op amp will be enough to make your constant current source unable to find...

OK, then in this case, we need to focus on the choice of op amp, right? This is a single power supply, right? And is the ideal op amp suitable for current simulation design?

maychang

fishsheep1919 posted on 2021-2-18 15:45 Okay, then in this case, we need to focus on the choice of op amp, right? This is a single power supply, and the current simulation design is ideal...

If you use a three-terminal ideal op amp for simulation, there is no restriction on the power supply. Assume it is a dual power supply and the power supply voltage is infinite.

If it is a five-terminal ideal op amp, then there will be limitations on the output voltage of the op amp, but there will be no offset temperature drift, etc.

chunyang

fishsheep1919 posted on 2021-2-18 15:43 It's like this, I'm a student, this is my first time to come into contact with this kind of thing, and then the teacher asked me to make a constant current source to power the accelerometer. Our teacher is very interested in this...

Then you should understand what Maychang meant by the leakage current and the parameters of the op amp that have a significant impact on the accuracy? I always feel that you are nitpicking, and there is no "alternative approach" here, and it has nothing to do with simplicity. The first thing you should solve is to understand what Maychang meant, that is, what determines the accuracy and what parameters affect it.