Home > transistors > What does this op amp/transistor circuit do?

What does this op amp/transistor circuit do?

July 12Hits:0
Advertisement

Consider this circuit:

What does this op amp/transistor circuit do?

What does this op amp/transistor circuit do?

– Schematic created using CircuitLab

It's not clear to me what this circuit does. Here's what I think I understand:

  • Depending on the position of the pot, the voltage at the non-inverting input is between 9V and 10V
  • Because of negative feedback, the voltage at the collector of the transistor is also between 9V and 10V (or is it? It's not clear how the second power supply input is affecting the feedback cycle).
  • The current through the load will be at most 10 A (perhaps this is a current source?).

I'm a beginner and these things are very confusing to me. In addition to a theoretical understanding, I'm also interested in how real (ie. non-perfect) op amps might behave differently and the tradeoffs involved in picking the right op amp for this circuit.

Answers

The circuit is broken because it mistakenly implements positive feedback, where the intent is to have negative feedback. The more resistance you dial in with the potentiometer, to raise the voltage on the + input, the more the MJE transistor conducts. This causes current to increase through R3, which lowers the voltage at the bottom of R3. This lowered voltage is fed back to the - input of the op-amp, and has the effect of increasing the output!

In effect, it's a double negative: R3 is the load resistor of an inverting gain stage, and this inverted output is fed to the inverting input of the op-amp. Inverted output fed to an inverting input is positive feedback.

The proper approach to take the feedback signal simply from the output of the op-amp (classic voltage follower/buffer). This buffer then simply drives the base of the transistor, implementing a classic emitter-follower to increase the current driving ability. R3 is not necessary.

Alternatively, the feedback signal can be taken from the node where the transistor's emitter meets the load (top of the load). That topology will then eliminate the VBE voltage drop, since it moves the VBE drop into the feedback loop. The consequence is that the load voltage will then closely follow the voltage on the op-amp's + terminal, rather than be a VBE drop lower.

Concretely, here is a version of the circuit modified with the above feedback topology, and also cleaned up to simulate. I got rid of the series resistor from the potentiometer so we can vary the voltage from 0 to 10V (by varying the "k" parameter of the pot from 0 to 1). The role of the load is played by R4.

What does this op amp/transistor circuit do?

What does this op amp/transistor circuit do?

– Schematic created using CircuitLab

If we move the feedback line to the original location, then the DC simulation shows that the circuit latches up with the transistor open.

Even if R3 is made substantially larger, the hypothesized positive feedback action doesn't happen precisely as described, but rather as follows: When power is applied, no current flows through R3, and so the - input of the op-amp is held at the 10V power rail. The + input cannot rise above this, and so the op-amp output is driven low. In this manner, the positive feedback keeps the transistor in a cut-off state. If we turn the potentiometer all the way up, it's more interesting: then the inputs are both nominally at 10V. The actual behavior will depend on their precise values, which are determined by bias currents and leakage through the cutoff transistor.

In order to analyse an op-amp circuit as an amplifier, certain assumptions must be made. One of those assumption is that negative feedback is present.

But, in this circuit, negative feedback is not present.

To see this, imagine that, for some reason (noise or some other disturbance), the voltage at the output of the op-amp increases (place an up arrow at the output of the op-amp).

Now, if negative feedback is present, the effect of this disturbance will be to make the voltage at the output of the op-amp to decrease. If this is not so, you do not have negative feedback.

So, trace this disturbance 'round the loop. Since the output of the op-amp is connected to the base of the transistor, the increase in base voltage will result in a decrease in collector voltage (place a down arrow at the collector).

The inverting input of the op-amp is connected to the collector (place a down arrow at the inverting input of the op-amp).

But, since this is the inverting input, a decrease in voltage there results in an increase in the op-amp output voltage.

In summary, a disturbance that increases the output voltage of the op-amp will result in feedback that tends to amplify that disturbance. This is the sign of positive feedback.

So, this circuit is not stable; any disturbance will be amplified rather than attenuated. I don't believe this is the desired operation. It's likely that the inverting input of the op-amp should be connected to the emitter of the transistor.

Looking at the circuit, I would think that by swapping the + and - inputs of the opamp you get a current source. Haven't tried the circuit though!

To analyse stability, use a thought experiment: what happens when I raise the voltage on the plus input. As a result, the output of the opamp will be raised as well. If this raise leads to a raise on the - input, the circuit can be stable, as the difference between the inputs is reduced by the action of the opamp.

In the original circuit, a raise on the + input leads to a raise on the output, which leads to more current through the transistor, which leads to more drop over R3, which leads to a drop in the - input. That circuit is unstable. Swap the inputs, and you have a nice current source.

For currents near zero, the opamp must be capable of handling inputs near the rail voltage, not all opamps are capable of this.

The circuit as drawn is a threshold detector.

It is component specific as there is a race condition at power on that might go either direction depending on the specific components and possibly even the layout. To make it start more reliably, put a small capacitor across BC.

The circuit has no reset as drawn so it can only be triggered once per power cycle. A N.C. switch in the feedback would allow resetting.

If the load is more than about an amp, the resistance has to be adjusted.

Related Articles

  • bjt transistor circuit - finding the resistances

    bjt transistor circuit - finding the resistancesMay 16

    I have been given the followign circuit diagram: and have been asked to compute \$R_2\$ and \$R_C\$ such that, at the Q-point, the circuit will behave as follows: \$V_{CE}=5\:\mathrm{V}\$ \$V_{BE}=0.7\:\mathrm{V}\$ \$I_C=2\:\mathrm{mA}\$ \$\beta=100\

  • Transistor circuits: NOR and short

    Transistor circuits: NOR and shortNovember 30

    I've two short questions regarding circuits with a transistors or diodes: 1) The following circuit with two diodes: Does it produce an electrical short for B=1 (5V) and for B=0 there is V[out]=1? Is this correct? 2) Does the following transistor circ

  • Transistor circuit to toggle motor on/off not working (Arduino)

    Transistor circuit to toggle motor on/off not working (Arduino)July 28

    I've been trying to create a simple transistor circuit with a 2N3904 that will toggle a geared DC motor on and off without drawing power from the arduino - I've followed the schematic seen in the 2nd image. (Breadboard view) (Schematic) The circuit i

  • Transistor Circuit question

    Transistor Circuit questionDecember 28

    I'm trying to solve a transistor problem, but I'm stuck on the I2. Is I2 equal to I0 ? The question is: The transistor circuit in Fig. 3.45 has β = 80 and VBE = 0.7 Find V0 and I0 . Answers : 12V, 600 mA. My equations: For \$I_1\$, \$-1+120 \mathrm{k

  • Solving transistor circuits

    Solving transistor circuitsJuly 27

    Solving transistor circuits is becoming very confusing. Is there a model for every topology? For example, I do not quite understand this answer: I thought \$I_{E}\$ and \$I_{C}\$ were almost equal so why is it that when you divide \$V_{CC}\$ with bot

  • Understanding transistor circuits

    Understanding transistor circuitsAugust 12

    I have been working my way through The Art of Electronics by Horowitz and Hill and I've just started the chapter on Transistors. I am having some difficulty in understanding this transistor circuit: The text explains that "When the switch is closed,

  • Simple NMOS transistor circuit output impedance

    Simple NMOS transistor circuit output impedanceOctober 6

    I am having trouble figuring out the best way to determine the output impedance of this simple NMOS transistor circuit: Both transistors are equal, and the body effect is ignored. What method should I use to find the output impedance? Should I find t

  • Why does my transistor circuit produce a wave?

    Why does my transistor circuit produce a wave?March 14

    Could some one explain to me how my circuit works? I accidentally hooked this up while trying to work out an amplifier. I'm thinking there's something like RC resonance going on, but I'm not sure. Please help? Thanks. --------------Solutions---------

  • Find Vo in an op-amp - transistor circuit

    Find Vo in an op-amp - transistor circuitApril 1

    I have a schematic with this system and can't find out how it works: – Schematic created using CircuitLab How do I find Vo in this network? Please explain it "for dummies". --------------Solutions------------- The trick to remember with linear o

  • Single LED single transistor circuit not working

    Single LED single transistor circuit not workingMay 25

    I was making a circuit with 1 LED and 1 transistor. pretty primarily i was trying to use the transistor as a normal switch. Here is the diagram below. So, this is an NPN transistor and i connected the -ve part of the D1 (transistor) with the -ve part

  • What is wrong with my L7805CV + BUF 654 NPN transistor circuit

    What is wrong with my L7805CV + BUF 654 NPN transistor circuitJune 9

    I really can't understand why this circuit get so hot and it does not turn on my LED strip (2 x 3 RGB leds). I use a L7805CV voltage regulator, a BUF 654 NPN transistor. I have check the regulator and it works fine when it's not connected to the tran

  • What does this op amp/transistor circuit do?

    What does this op amp/transistor circuit do?July 12

    Consider this circuit: – Schematic created using CircuitLab It's not clear to me what this circuit does. Here's what I think I understand: Depending on the position of the pot, the voltage at the non-inverting input is between 9V and 10V Because of n

  • What does this op amp/transistor circuit do? (revised circuit)

    What does this op amp/transistor circuit do? (revised circuit)July 12

    – Schematic created using CircuitLab I asked a question yesterday about the circuit above, but my notes showed the op amp terminals mistakenly swapped and the circuit in my original question displayed positive feedback. After studying it some time, I

  • NPN transistor circuit connection, what determines which region it is at?

    NPN transistor circuit connection, what determines which region it is at?September 26

    I know that for an NPN transistor to be in the active region, the base emitter should be forward biased and the base collector should be reverse biased while for the NPN transistor to be in the saturation region, the base emitter and the base collect

  • Minimum input voltage for this Transistor circuit

    Minimum input voltage for this Transistor circuitJanuary 26

    I have a circuit below, which I am using in a Coil Gun to amplify a pulse which is generated when a projectile passes close to a coil. I was able to get it to work, however I had the question of what is the maximum voltage that I can pass as an input

  • Input resistance of a transistor circuit

    Input resistance of a transistor circuitMarch 18

    I have a question from the razavi's microelectronics book: I'm puzzled because in the solution provided, the first line states that: "We know that the input resistance Rin = R1||R2||\$r_π\$." While I understand if I transform the circuit's base

  • Voltage between collector and emitter (in transistor circuit) [2N3904]

    Voltage between collector and emitter (in transistor circuit) [2N3904]May 7

    Let's say we have the following circuit with silicon transistor 2N3904 (the transistor has \$\beta = 125\$): I am trying to calculate the DC voltage between the collector and the emitter. One of the solutions I found is the following: $$Ι_b = \frac{V

  • Programmable Unijunction Transistor Circuit

    Programmable Unijunction Transistor CircuitJanuary 21

    I am currently teaching my self electronics and have a question about PUTs. I am wondering why there has to a resistor on both sides of the PUT in this circuit: Please note that I have already read the question regarding the Make Electronics Experime

  • Need help understanding Transformerless Push-Pull transistor circuits

    Need help understanding Transformerless Push-Pull transistor circuitsMarch 9

    I am new to electronics and I am currently trying to learn a little more about how NPN and PNP transistors work. While doing some research, I stumbled across a circuit design called the Transformer-Less Push-Pull circuit. The schematic that describe

  • Simple transistor circuit

    Simple transistor circuitMay 4

    I have been studying transistors and decided to try my knowledge in my lab to find out that nothing works as I thought it would.. I had a small lamp 12V and a handful of 2N3904 transistors, along with a 12V DC power supply. Attached to the base of th

Copyright (C) 2017 ceus-now.com, All Rights Reserved. webmaster#ceus-now.com 14 q. 0.363 s.