We can use differential amplifier to amplify the difference between two input signals and compare this output with a threshold value(either in circuit or with microcontroller like Arduino Nano), to take action if the it is over or below the threshold. Application example includes: compare indoor and outdoor temperatures to control heating/cooling systems, monitor the temperature difference between two critical points in a circuit to detect potential overheating, use this setup in scientific experiments to measure temperature gradients and others.
Here we will show how to build a simple basic differential amplifier with 2N3904 BJT transistor. You can use similar transistors like BC547, 2N2222 etc. Here we will design a differential amplifier with bipolar transistors for a Temperature Difference Indicator with LM35, BJT Differential Amplifier, and Arduino Nano. The circuit diagram is as shown.
Resistor Value calculation
When designing a differential amplifier with BJTs and interfacing it with the LM35 temperature sensors, choosing the appropriate resistor values is crucial for proper operation. Here's how you can select suitable values for the common emitter and collector resistors, and whether you need a base resistor between the LM35 output and the transistor base.
1. Common Emitter Resistor (Re)
The common emitter resistor helps set the gain and stability of the differential amplifier.
- Typical Value: A value between 1kΩ and 4.7kΩ is usually a good starting point.
- Impact on Circuit: A lower resistor value (e.g., 1kΩ) will allow more current through the transistors, increasing the gain but reducing the input impedance. A higher value (e.g., 4.7kΩ) will decrease the gain but improve input impedance and reduce noise.
2. Collector Resistors (Rc)
The collector resistors set the gain of the amplifier and the voltage swing at the output.
- Typical Value: A value between 4.7kΩ and 10kΩ is common.
- Impact on Circuit: The collector resistors determine the voltage drop across the transistors. Larger values (e.g., 10kΩ) will give a higher voltage gain but may reduce the headroom for the output voltage swing. Smaller values (e.g., 4.7kΩ) will provide a lower gain but a wider output voltage range.
3. Base Resistor between LM35 Output and Transistor Base
Whether you need a base resistor depends on how the circuit is designed:
Direct Connection: If you connect the LM35 output directly to the transistor base without any resistor, the base-emitter junction may draw more current than necessary, possibly affecting the accuracy of the LM35 output and loading the sensor.
Base Resistor: Adding a base resistor can limit the current into the base of the transistor, protecting both the LM35 and the transistor.
- Typical Value: A resistor between 10kΩ and 100kΩ is often used.
- Function: This resistor limits the current into the base, ensuring the transistor operates in the linear region and prevents excessive current draw from the LM35.
Final Circuit Design Tips
- Start with Standard Values: You can begin with Rc = 10kΩ, Re = 2.2kΩ, and base resistors of 47kΩ.
- Adjust Based on Performance: If the gain is too low, try decreasing Re or increasing Rc. If the LM35 readings seem affected, adjust the base resistor value.
This approach provides a good balance between performance and stability for your temperature difference indicator circuit.
