UJT Relaxation Oscillator with Op-Amp Squarer

The UJT below is configured to produce a sawtooth wave at a frequency of 104 Hz. The capacitor begins to charge through the 4.7 kOhm resistor when the circuit is initially powered up. The current suddenly flows from the UJT emitter (E) to the UJT Base (B1) when the voltage across the capacitor rises to a certain point. This will produce a voltage across the 100 ohm resistor which is the same as the output of the UJT relaxation oscillator on the Emitter. This is the figure of the circuit;

The sawtooth waveform is applied to op amp pin 3 and the voltage divider is applied to op amp 2. The op amp produces a positive going output on [in 6 when the sawtooth is higher than the reference from the divider of 2.5 volts. The op amp acts as a comparator. [Circuit's schematic diagram source: Chuck Bolin Electronic Project].

Passive Baxandall Tone Control (2 Band Equalizer)

Variation of the famous Baxandall circuit is shown in the passive circuit in figure 1. Smoothly increasing +- 6 dB/octave slope of boost or cut is the feature of this circuit. Although the “shelves” are outside the audible range with these component values, the bass and treble filters have a shelving response. The threshold and shelving frequencies is predicted by the filter equations in figure 1a.The wiper shorts out the .033uf capacitor, when the bass control is rotated for maximum boost. A frequency dependent voltage divider that determines the shelf frequency of the boost are formed by R3 and C4. The wiper bypasses the 10K resistor. C1 and R2 form a high pass filter, when the treble control is set for maximum boost. This is the figure of the circuit;

The circuit uses commonly available parts to simplicity. Radio Shack sold even the 100k log taper pots, which are usually hard to find, as part number 271-1732 (in a stacked configuration for stereo). The pot will have about 10K on one side and 90K on the other at the midway point.The side with 10K parallels R2 for the treble control. The side with 10K parallels C4 for the bass control. Note that this circuit could be divided for an individual bass or treble control (R5 may then be omitted – it helps isolate the bass from the treble circuit when the two are put together). To avoid loading the network and affecting the response curves, the passive Baxandall must see a low impedance source and drive a high impedance load.

Op Amp Voltage Regulator Circuit

This is a simple voltage regulator circuit that employs an operational amplifier (op-amp).  As its name implies, this circuit accepts an unregulated voltage input (i.e., a fluctuating input voltage), and provides a regulated voltage output (a stable output voltage that remains at or very close to its intended output level).  The unregulated input voltage must be higher than the desired output level by a sufficient margin in order to achieve 'effective' regulation. This is the figure f the circuit;

The zener diode Vz acts as a voltage reference for the circuit, and is fed into the non-inverting input of the operational amplifier.  The voltage divider formed by R1 and RF sets the voltage level of the inverting input of the op amp, which is basically a feedback from the circuit output to the op amp.  The NPN transistor is used to boost the output current of the circuit. The voltage at the non-inverting input of the op amp is pegged at the zener voltage, while the voltage at the inverting input is always a fraction of the output voltage as defined by RF and R1.  When the output exceeds the set level, the inverting input voltage exceeds that of the non-inverting input, causing the output of the op-amp to go 'low'.  This turns off the NPN transistor, causing the output voltage to dip.  When the output goes below the set level, the reverse happens, i.e., the op-amp's output goes 'high', causing the NPN transistor to turn on and pull the voltage up.

         

Thus, this circuit works by turning off the transistor when the output voltage is too high and turning it on when the output is too low.  This balancing act happens continuously, with the circuit reacting instantaneously to deviations in the output voltage.  Resistor RF is adjusted to set the desired output voltage of the circuit.  The zener diode needs to be replaced by a voltage reference IC if a more stable and more precise output is required.

OP90 4 mA to 20 mA Current Loop Transmitter Circuit

This is a circuit for current loop circuit. An output of 4 mA to 20 mA that is linearly proportional to the input voltage is provided by the current transmitter on figure below. Line rejection is 0.0005%/volt and linearity of the transmitter exceeds 0.004%. This is the figure of the circuit;

The REF-02EZ provides biasing for the current transmitter. The output current is regulated by OP90EZ to satisfy the current summation at the noninverting node:

Iout = 1/R6((Vin.R5)/R2 + (5V.R5)/R1)

Iout = (16/100 ohm)Vin + 4mA

That’s will give a full-scale output of 20 mA with a 100mV input. By adjusting R2, we can provide a offset trim and adjustment of R1 will provide a gain trim. Since the non-inverting input of the OP90 is at virtual ground, these trims do not interact. The input voltage spike will be prevented by D1 (the Schottky diode) from pulling the non-inverting input more than 300 mV below the inverting input. Without diode, such spikes could cause phase reversal of the OP90 and possible latch-up of the transmitter. This circuit has compliance from 10V to 40V. For transducer excitation, the voltage reference output can provide up to 2 mA. [Circuit diagram source: Analog Application Note]

Enabling 3 Phase Motor to Operates with Single Phase Supply

The capacitor has been use for decades to operate 3-phase motors on single-phase power. Two single-phase wires are connected to two of the inputs on a 3 phase motor on this method. Then, the capacitor is connected to one of the single -phase inputs and the third leg of the motor. This is the figure of the circuit;

The voltage is allowed to be displaced in time from its parent voltage by phase shift through the capacitor. Voltage distinct from the 2 single-phase lines is the result. The motor will operate if the capacitors value-it’s ability to process electrical current- is sufficient. 6 times as much current to start as it does to run is required by the motor so a static-capacitor phase converter must have some means of switching a large group of capacitors in and out during motor starting. Below is a typical unit uses a potential-type motor starting relay (pirated from a single-phase motor) to regulate the larger start capacitor, while a smaller (in value) capacitor provides continuous power to run the motor. The potential relay removes the start capacitor from the circuit as the motor speed increase, and the motor operates.

BTL Mono Amplifier with DC Volume Control Circuit

This is the circuit for BTL  (bridged tied load) mono amplifier with DC volume Control circuit. This circuit uses TDA7052A/AT that is suitable not only for monitors and TV but also battery fed portable radios and recorders. The difference between conventional DC volume circuits and TDA7052A/AT the DC volume control is there is no coupling capacitor is needed in TDA7052A/AT the DC volume control to maintain the low offset voltage. This is the figure of the circuit;

Besides that, the TDA7052A/AT the DC volume control requires low supply. The advantages of BTL principle are it has the ripple’s frequency on the supply voltage is twice the signal frequency and the supply current peak is lower. To save the cost, a smaller capacitor can be used. The BTL principle can be implemented in portable application that will decrease supply voltage but increases the output power. This amplifier has maximum gain of 35.5 dB. The mute mode is when the DC volume control voltage is below 0.3V. This circuit also equipped with thermal protection. The gain will be decreased when temperature reach +150 C. [Circuit's schematic diagram source: nxp.com]

Analog Tremolo (Guitar) Sound Effect Circuit

This is the circuit for the tremolo effect produce an amplitude modulated signal. Unlike vibrato effect that produce frequency modulated, this tremolo effect will produce the same effect as quickly turning up-ad-down your volume control repeatedly. This is the figure of the circuit;

Q1 is general small signal audio transistor, you can use almost any type of NPN small signal transistors. MFC3304P is a voltage controlled amplifier IC with negative control, it might be difficult to find these days, but I think you can replace with general OTA (operational trans-conductance amplifier), with the appropriate biasing circuit off course. Just bias the current control of an OTA through a resistor, and add the C4 modulator signal at the control pin of the OTA.