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 ...

This Discrete Sliding Tone (Frequency Ramp) Doorbell circuit produces a low tone that will slide up to higher frequency. The equivalent total resistance connected between the base of Q1 and ground (Rbg) , and coupling capacitor  C1  determines the AF oscillator’s frequency. The resistance (Rbg) is equal to (R2+R1)R3.   Here is the schematic diagram of the circuit. The R2 is used to set the initial bias condition, adjusted to produce a pleasant low starting frequency doorbell tone. D1 will start to conduct when Capacitor C3 charge through R6 until it reaches D1 bias  voltage level.  Then the value of Rbg is paralleled by R4 and D1, and R5-D2-D3, and the values of diode’s equivalent resistance is gradually decreased as the C3 voltage ramp up.  This decreasing resistance value make the output tone slides up in frequency.  Two different diode path is provided to extend the linear area of diode conduction transition slope. With two path with different ...