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I am working on a reference voltage circuit and I am having trouble when selecting the appropriate resistor and capacitor values. The circuit involves OP07 operational amplifiers and transistors (2N2222 and 2N2907) to generate positive and negative adjustable voltage references.

Here is the circuit and its functionality:

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Circuit Description:

This is only one stage of a complete circuit. The next stage is expected to consume a maximum of 100mA from both VREF+ and VREF-.

  • OP07 Operational Amplifiers: Configured as voltage followers (buffers) to generate stable reference voltages.

  • 2N2222 (NPN) and 2N2907 (PNP) Transistors: Used to amplify the current capacity of the reference voltages.

  • Diodes (1N4148): Placed in series with the base of the transistors to compensate for the base-emitter voltage drop and to extend the output voltage range, since OP07 is not rail-to-rail.

  • R6 and R7: Resistors used to bias the diodes correctly and to set the base current of the transistors.

  • R8 and R9: Dummy loads connected to the output.

  • C1 and C2: Capacitors used for stabilizing the outputs.

Operation:

The OP07 amplifiers are used as voltage followers to provide stable VREF+ and VREF-. The feedback loop of the opamps includes the V_BE voltage of the transistors, so the voltage at the emitter is expected to be the same as the input voltage of the voltage follower. The transistors (2N2222 for VREF+ and 2N2907 for VREF-) are used to increase the current capacity of the reference voltages.

The diodes (1N4148) in series with the base of the transistors compensate for the 0.7V base-emitter drop, ensuring accurate voltage references, and also compensate for the fact that the OP07 is not rail-to-rail, allowing full use of the opamp's output voltage range. R6 and R7 are crucial for biasing the diodes correctly and setting the appropriate base current for the transistors.

R8 and R9 act as dummy loads to simulate the presence of a real load. C1 and C2 are used to stabilize the reference voltage outputs, minimizing noise and fluctuations. I am considering using electrolytic capacitors for C1 and C2.

Questions:

  • Selecting R6 and R7:

How do I accurately calculate the values for R6 and R7 to ensure proper transistor operation and stable reference voltages?

The base current 𝐼𝐵 should be sufficient to drive the transistors without exceeding their maximum ratings. Given an expected collector current 𝐼𝐶 of approximately 100mA, how should I determine the exact resistor values?

  • Selecting R8 and R9:

These are dummy loads and I am considering using 1MΩ resistors. Is this value appropriate, or should I consider a different value to avoid potential issues?

  • Capacitor Selection (C1 and C2):

I am considering using electrolytic capacitors for C1 and C2. What considerations should I consider when choosing capacitors for filtering and stability in this circuit? Specifically, what capacitance values and types are recommended to ensure minimal noise and maximum stability for VREF+ and VREF-?

Any guidance or references to similar designs would be greatly appreciated.

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  • \$\begingroup\$ Why not go for a voltage reference IC with a subsequent voltage buffer that has current capability you need? \$\endgroup\$
    – Designalog
    Commented Jul 2 at 14:16
  • \$\begingroup\$ I guess you are right, maybe some LM317 and LM327 would do the trick, but I think this circuit is capable of buffering a voltage coming from a DAC for example, and amplifying its current capabilities, so In a more elaborated device, the voltage references would be able to be digitally controlled. \$\endgroup\$ Commented Jul 2 at 20:55

1 Answer 1

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To build a voltage reference that depends on BJTs without feedback, diodes and potentiometers is not a good idea. This will not offer good voltage stability.

You can select R6 and R7 based on the BJT datasheet. Look for beta at 100mA and you will get the required max. base current, e.g. 1mA for beta=100. Then look for the Vce voltage drop at 100mA, e.g. 1.3V. Subtract Vbe=0.7V and you get the voltage drop on R6,R7, e.g. 0.6V. So you get R6,R7 = 0.6V / 1mA = 600 Ohms.

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  • \$\begingroup\$ But this circuit does indeed depends on the opamps negative feedback \$\endgroup\$ Commented Jul 2 at 16:18
  • \$\begingroup\$ Collector-Emitter is missing feedback. \$\endgroup\$ Commented Jul 2 at 17:01
  • \$\begingroup\$ I'm sorry, I don't get why Colletor-Emmiter feedback is needed, the opamp is buffering the input voltage to the emitter voltage, the collector is always connected to the power supply, so the opamp should compensate the base voltage and current for it to satisfy the feedback requirements, what am I missing? The potentiometers that feed the voltage follower could be replaced by something else like a DAC. It was just a simpler way to show the working of the circuit, also doesn't the base current come from the opamp? \$\endgroup\$ Commented Jul 2 at 21:01
  • \$\begingroup\$ You are right, the BJT is completely controlled. But the base current comes from the resistor(s). The opamp just attenuates/reduces the base current. \$\endgroup\$ Commented Jul 3 at 7:54
  • \$\begingroup\$ Oh, I think I get it now, R6 and R7 bias something like the transistor's DC operating point, so the operational amplifier should only compensate for the feedback requirements, which will only change when the input signal varies. \$\endgroup\$ Commented Jul 4 at 22:08

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