Rongge

You can use three discrete transistors to build an operational amplifier with an open-loop gain greater than 1 million (Figure 1). You bias the output at approximately one-half the supply voltage using the combined voltage drops across zener diode D 1 , the emitter-base voltage of input transistor Q 1 , and the 1V drop across 1-MΩ feed-back resistor R 2 . Figure 1 . This ac-coupled inverting op amp has an open-loop gain of 1 million. R 1 and R 2 set a closed-loop gain of −10. Resistor R 3 and capacitor C 1 form a compensation network that prevents the circuit from oscillating. The values in the figure still provide a good square-wave response. The ratio of R 2 to R 1 determines the inverting gain, which is −10 in this example. You can configure this op amp as an active filter or as an oscillator. It drives a load of 1 kΩ. The square-wave response is good at 10 kHz, and the output reduces by 3 dB at 50 kHz. Set the 50-Hz low-frequency response with the values of the input ...

How make a Simple Siren with Four Transistors. This circuit requires only small 6V power from the speaker to generate sound two tones. Simple Siren with Four Transistors Circuit Diagram The circuit is more than simple. The first two transistors (left) are responsible for forming a flip-flop. This means that the 10K resistor will have a pulsed signal. The 4.7μF capacitor is charged and discharged in this signal are almost pulsating. Applied this to the oscillator (formed by the other two transistors) we obtain the desired sound output. The speaker can be any 8 ohm 1 watt the type used in portable radios.