How Does an Opamp Circuit with a Temperature Sensor Work?

[likephysics]

Homework Statement

I am trying to figure out how this ckt works. But no idea, where to even begin.
see attachment tp.png

Homework Equations

The Attempt at a Solution

Q1 is diode connected. The Vbe temperature dependence is amplified to indicate the temperature. -2mV/ deg C
Gain is about 50.
I simplified the ckt to the one in pdf attachment.
The -15v is added to inverting terminal and -13.85v is added to non-inverting terminal.
Assuming Vbe is 0.64v at room temp, -13.85 and -0.64v add up to -14.5v
The difference between invering and non-inverting terminals is 0.5v.
Can't go any further.

 

[berkeman]

Homework Statement

I am trying to figure out how this ckt works. But no idea, where to even begin.
see attachment tp.png

Homework Equations

The Attempt at a Solution

Q1 is diode connected. The Vbe temperature dependence is amplified to indicate the temperature. -2mV/ deg C
Gain is about 50.
I simplified the ckt to the one in pdf attachment.
The -15v is added to inverting terminal and -13.85v is added to non-inverting terminal.
Assuming Vbe is 0.64v at room temp, -13.85 and -0.64v add up to -14.5v
The difference between invering and non-inverting terminals is 0.5v.
Can't go any further.

Sweet! That circuit is from near the end of National Semiconductor's App Note AN-31, an Opamp Circuit Collection. Classic AN.

Well for one thing, with negative feedback, the voltage between the + and - inputs is most definitely not 0.5V.

You might have a look at National Semiconductor's AN-56 (which is in the same book as AN-31 if you have it, their "Linear Applications Handbook"). Figure 6 in AN-56 is an Electronic Thermometer opamp circuit, very similar to the one you posted. AN-56 has an explanation of how the circuit works, so that should help you figure out this AN-31 circuit.

Let us know what you figure out!

 

[likephysics]

Okay. That helped.
I simplified the ckt to the following.
A 1.2v reference for constant current biasing of temp sensor.
A -1.2v reference for zeroing.
I am not sure how the zeroing works. To zero a diode with +1.2v bias, you need another diode or a resistor with its resistance equal to diode resistance with a -1.2v bias. correct?
I don't know why a 5k pot is used.
I've attached the AN56 ckt and my simplified ckt pdf.

 

[likephysics]

Anyone?

 

[DeeNos]

First, let's break down the components in this circuit: an opamp, a temperature sensor, and a diode-connected transistor (Q1).

The opamp is a type of amplifier that takes an input signal and amplifies it to a larger output signal. It has two inputs, an inverting and a non-inverting terminal, and one output. The output is equal to the difference between the two inputs multiplied by the gain of the opamp.

The temperature sensor is a device that measures temperature and converts it into an electrical signal. In this circuit, the temperature sensor is likely a thermistor, which is a type of resistor that changes its resistance based on temperature.

The diode-connected transistor (Q1) is used to amplify the temperature signal from the thermistor. The voltage across the base-emitter junction of the transistor (Vbe) is known to have a temperature dependence of -2mV/deg C. This means that for every degree increase in temperature, the Vbe voltage decreases by 2mV. This change in voltage is amplified by the opamp to provide a larger output signal.

In this circuit, the -15V voltage is added to the inverting terminal of the opamp, while the -13.85V voltage is added to the non-inverting terminal. This creates a voltage difference of 0.5V between the two inputs, which is then amplified by the opamp to provide the output signal.

Overall, this circuit is designed to accurately measure and amplify temperature changes using the Vbe voltage of a diode-connected transistor. By understanding the function of each component and how they work together, you can better understand how this circuit operates.