Programmable amplifier that can be switched 8 times (3 bits) in the order of 1x, 2x, and 5x

Programmable amplifier that can be switched 8 times (3 bits) in the order of 1x, 2x, and 5x

Function of the circuit

The variable amplifier of the programmable amplifier that can set the magnification of 1, 10, 100, 1000 times is switched in units of 10 times, but the switching range is often too large. This circuit can be set to a maximum of 200 times in the order of 1, 2, and 5 times, and can use the analog multiplexer TC4051, which is extremely cheap.

The magnification is selected by a 3-bit positive logic input, and there are 8 possible switching options.

How the Circuit Works

This circuit obtains the required amplification factor by dividing the feedback resistor of the positive phase amplifier circuit. The 900 ohms and 9 kilo ohms in the divided resistors do not belong to the E series resistance value, and can be obtained by connecting two 1.8K and 18K resistors in parallel respectively. The 4051 type multiplexer has a level shift and a decoder, so the circuit can be simplified. However, the power supply voltage must be below positive and negative 7.5V, which is inconvenient to use. This circuit uses a Zener diode to form a positive and negative 6.8V power supply voltage. Then use resistor R3 to divide the output voltage of A1 by half, so that there will be no signal input higher than the power supply voltage. Because the amplification factor cannot be 1 under this condition, a 1/2 public voltage resistor of 3 is added to the input end of the OP amplifier.

When the amplification factor is 200 times, the circuit works at a gain of 400 times, which causes the loop gain of the OP amplifier to decrease and the frequency characteristics to deteriorate. Therefore, care should be taken when amplifying signals of tens of Hz.

Notes

Output protection circuit of OP amplifier

It is unclear what kind of signal is connected to the analog output terminal of the measuring instrument or the monitoring output circuit. Sometimes the output OP amplifier is damaged due to output short circuit or excessive input.

Figure 1 is an application example that does not require low output impedance. A current limiting resistor R0 is added to the circuit in series with the output terminal. However, there are certain problems when it is used as a high-precision voltage output circuit. Figure 2 is a practical output protection circuit. R1 is a current limiting resistor, and R2 is an output feedback resistor. Diodes D1 and D2 connected to the positive and negative V bypass the overvoltage.

The output end is often connected in parallel with some static capacitors, which sometimes cause oscillations. Capacitor C1 is used to stabilize these capacitive load capacitors.