Pre Amplifier Circuit for Oscilloscope

An oscilloscope amplifier with 20 dB voltage gain is provided by this circuit with a frequency range from 0.5 to 50 MHz. By increasing the value of the 0.05 uF capacitor or try removing the capacitor, we can extend the low frequency response of this circuit. This is the figure of the circuit;

A particularly small level of input noise is delivered by this circuit, measured at approximately 20uA over a bandwidth range at 15 MHz. By adjusting the gain potentiometer connected between pins 3 and 10, then adjust the 1-KΩ trimmer potentiometer for an exact voltage gain of 10 are the ways to calibrate the gain. This helps preserve the scale factor of the oscilloscope.

NPN Transistor Darlington Configuration

In the Darlington configuration, this emitter follower has a pair of transistors. The emitter current of one transistor becomes the base current of the second in this arrangement. The Darlington configuration acts like one transistor with a beta which is the product of the betas of the two transistors. This is the figure of the circuit;

Where high output currents are needed, they can be used. The Darlington configuration has a quite high input impedance. The resistor is commonly tied between the emitters to increase the speed switching because switching of the second transistor may be slow. Darlington pairs are available as single packages with resistor included.

Miniature Audio Oscillator Circuit

This circuit is designed as a pocket sized high performance audio oscillator. This circuit can operated using battery operated version was possible and could be made at very low cost as well by using one quad op-amp to provide the entire active circuitry. This is the complete figure for design circuit.

In the figure, there are only two control pots (RV1 and RV2) and two DPDT switches. The output level pot includes an on-off switch and is of logarithmic taper to allow easier setting at low (i.e. millivolt) levels. This pot is directly coupled to A4's output to minimize response errors, provided that the load impedance is constant or quite high compared to the output impedance provided by Miniosc. The frequency sweep control (RV1A/B) has a range of about 24:1 and in combination with the High-Low range switch having a 18:1 ratio, the audio band is covered (with the exception of the lowest octave) in two overlapping ranges. The possibility of a single sweep of the audio band without the range switch was tried out and later dropped in preference to the present design.

Types like the TL074, TL084, LF347 and LF444 and other quad op-amps with compatible pin outs are not recommended for use due to both the increased battery drain and reduced margin of minimum operating voltage. The TL064 is alone in having operation specified down to a plus and minus 3 volt supply. [Schematic’s circuit source: Phill Allison Notes].

Pierce XTAL Oscillator Circuit Using JFET

This circuit is conventional “Pierce” type oscillator that uses a JFET. The circuit uses fundamental mode crystals. It has decent performance and reliability if we use a low noise JFET. This is the figure of the circuit.


The feedback is controlled by the C1 Capacitance from drain to ground. Adjusting the frequency can be done by adjusting a shunt capacitance C2 across the crystal. The crystal works in parallel mode. This circuit is suitable where some crystals should be switched in and out to select the frequency, as there’s no tuning required.

Precision Relaxation Oscillator Circuit Using LM131

This is a circuit for precision oscillator that is the basic design using a voltage to frequency converter and based on op amp LM131. Basically, this IC, like any V/F converter, is a precision relaxation oscillator that generates a frequency linearly proportional to the input voltage. As might be expected, the circuit has a capacitor, CL, with a saw tooth voltage on it. This is the figure of the circuit.


The general description about this circuit is the circuit is a feedback loop that keeps this capacitor charged to a voltage very slightly higher than the input voltage, VIN. If VIN is high, CL discharges relatively quickly through RL, and the circuit generates a high frequency. If Vin is low, CL discharge slowly, and the converter puts out a low frequency. When CL discharges to a voltage equal to the input, the comparator triggers the one-shot. The one-shot closes the current switch and also turns on the output transistor. With the switch closed, current from the current source recharges CL to a voltage somewhat higher than the input. Charging continues for a period determined by RT and CT. At the end of this period, the one-shot returns to its quiescent state and CL resumes discharging. [Schematic’s source: National Semiconductor, Inc].

Negative Resistance Oscillator Circuits

This is a circuit of negative resistance circuits. All of the preceding circuits rely on RC time constants to achieve resonance. LC combinations can also be used and offer good frequency stability, high Q and fast starting. In the circuit, signal input is control and amp using op amp LF353. This is the figure of the circuit.


In this circuit, a negative resistance configuration is used to generate the sine wave. The Q1-Q2 pair provides a 15 μA current source. Q2's collector current sets Q3's peak collector current. The 300 kΩ resistor and the Q4-Q5 LM394 matched pair accomplish a voltage-to-current conversion that decreases Q3's base current when its collector voltage rises. This negative resistance characteristic permits oscillation. The frequency of operation is determined by the LC in the Q3-Q5 collector line. The LF353 FET amplifier provides gain and buffering. Power supply dependence is eliminated by the zener diode and the LF353 unity gain follower. This circuit starts quickly and distortion is inside 1.5%.

Wien Bridge Oscillator Circuit

This is a circuit that is known as wien bridge oscillator circuit. The circuit has positive and negative feedback loop. This circuit is work with control by op amp. This is the figure of the circuit.


The circuit oscillates at a frequency determined by the RC time constant at frequency and produces a sinusoidal waveform at the output voltage Vout. In many cases this circuit is used as sine wave generator which is using rail to rail op amp. [Schematic’s diagram source: Advanced Linear Devices, Inc]

Phase Shift Oscillator Circuit Using LM386

This is a design circuit of a simple inexpensive amplitude stabilized phase shift sine wave oscillator which requires one IC package, three transistors and runs off a single supply. This circuit is combination with the RC network comprises a phase shift configuration and oscillates at about 12 kHz. The remaining circuitry provides amplitude stability. Here’s the schematic figure of the circuit.


The high impedance output at Q2's collector is fed to the input of the LM386 via the 10 μF-1M series network. This circuit is using op amp LM386 causes it has fixed gain of 20. The 1M resistor in combination with the internal 50 kΩ unit in the LM386 divides Q2's output by 20. The positive peaks at the amplifier output are rectified and stored in the 5 μF capacitor. This potential is fed to the base of Q3. Q3's collector current will vary with the difference between its base and emitter voltages. Since the emitter voltage is fixed by the LM313 1.2V reference, Q3 performs a comparison function and its collector current modulates Q1's base voltage. Q1, an emitter follower, provides servo controlled drive to the Q2 oscillator.

Free Running Oscillator Circuit

This is a circuit for oscillator using astable mode operation. The basic oscillatior is using 555 timer IC. This circuit is also give mode free running oscillator. This is the figure of the circuit.


Operation of the circuit is begin, when initialy by capacitor C charged towards 2/3 V+ with Ra and Rb. When voltage on C reaches that threshold level, the discharge output in pin 7 is turning on to discharging C. Using CMOS 555 timer IC is a very wide frequency at very low of voltage spikes and dissipation can be achieved. Selections of values the Ra and Rb is limited by input leakage specification at time in pin 7, 2, and 6.

Using TLV2471 for Wein - Bridge Oscillator

The Wien - Bridge oscillator is one of the simplest and best known oscillators and is used extensively in circuits for audio applications. The figure in the below shows the basic Wien bridge circuit configuration. On the positive side, this circuit has only a few components and good frequency stability. The major drawback of the circuit is that the output amplitude is at the rails, which saturates the op-amp output transistors and causes high output distortion.

The gain, A, of the negative feedback portion of the circuit must then be set. RF must be set to twice the value of RG to satisfy this condition. The op amp in the figure is single supply, so a dc reference voltage, VREF, must be applied to bias the output for full-scale swing and minimal distortion. Applying VREF to the positive input through R2 restricts dc current flow to the negative feedback leg of the circuit. VREF was set at 0.833V to bias the output at the mid rail of the single supply, rail-to-rail input and output amplifier, or 2.5 V, (see reference in the data sheet of oscillator). VREF is shorted to ground for split supply applications.

The Wien – Bridge oscillator circuit is shown in the figure, with component values selected to provide an oscillation frequency of 0 = 2f0, where f0 = 1/(2RC) = 1.59 kHz. The circuit oscillated at 1.57 kHz, caused by varying component values with 2.8% distortion. This high value results from the extensive clipping of the output signal at both supply rails, producing several large odd and even harmonics. The feedback resistor was then adjusted 1%. Figure 9 shows the output voltage waveforms. The distortion grew as the saturation increased with increasing RF, and oscillations ceased when RF was decreased by a mere 0.8%.

Sine-Wave Oscillator

Oscillators are circuits that produce specific, periodic waveforms such as square, triangular, saw tooth, and sinusoidal. They generally use some form of active device, lamp, or crystal, surrounded by passive devices such as resistors, capacitors, and inductors, to generate the output. There are two main classes of oscillator: relaxation and sinusoidal. Relaxation oscillators generate the triangular, saw tooth and other non sinusoidal waveforms and are not discussed in this note. Sinusoidal oscillators consist of amplifiers with external components used to generate oscillation, or crystals that internally generate the oscillation. The focus here is on sine wave oscillators, created using operational amplifiers op amps.

Op-amp sine-wave oscillators operate without an externally-applied input signal. Instead, some combination of positive and negative feedback is used to drive the op amp into an unstable state, causing the output to cycle back and forth between the supply rails at a continuous rate. The frequency and amplitude of oscillation are set by the arrangement of passive and active components around a central op amp.

Oscillators are created using various combinations of positive and negative feedback. The basic negative feedback amplifier block diagram with a positive feedback loop added is one of the type of oscillator. When positive and negative feedback are used, the gain of the negative feedback path is combined into a single gain term (representing closed-loop gain). You can look at the figure below. The figure is show Positive and Negative Feedback Loops.

Figure;

Microphone Circuit Test Oscillator

This circuit would be mounted in a small plastic or preferably metal box, with a 9V battery, level control, a male XLR connector (same as on a microphone) and a switch. Current drain is low, since the circuit only uses one dual op amp. There is no need for a high quality device, and a 1458 is all that is needed.

The first stage is the oscillator itself. This is a simple three stage phase shift oscillator - a circuit that is remarkably uncommon - which is to say I have never seen it used elsewhere. I designed it for another project a few years ago, and I don't understand why it is not in any op amp application notes. Maybe I invented a new circuit.

If you want to tune it, you can use a 50k pot instead of R1. I suggest that if tuned, set it to A-440 Hz. Frequency stability is not wonderful, and it changes by a few Hertz as the battery discharges, but this is unlikely to cause problems - it is a test oscillator, not a tuning standard. As shown, frequency will be about 430Hz, depending on the accuracy of the capacitors. The phase shift network (R1-C1, R2-C2 and R3-C3) serves two purposes. First (and for an oscillator, most importantly), it shifts the phase of the output signal so the feedback is positive, causing oscillation. Secondly, since it is a three stage filter, it attenuates the signal and filters the output square wave so the signal at pin 2 is a reasonable sine wave. Distortion (if you really care) is about 3% or so - I didn't measure it this time, but I recall having done so before.

The second stage is the output buffer, and the signal is simply split to supply the two mic leads. The metal case should be connected to pin 1 (earth) on the XLR connector. The output level control must be a linear type, as the circuit loading will create a good approximation to a log pot. Maximum output into a typical microphone input will be about 100mV (unloaded oscillator output on mine was 140mV). It is not much to it - the whole circuit can be built on a small piece of ferro board, and the battery, pot and XLR connector will take up far more room than the oscillator. There is no LED indicator for power, as this would draw more current than the circuit. To prevent accidentally turning it on, a slide switch is suggested. They are a pig to mount compared to a toggle switch, but are much less easily bumped. If you can get a pot with a switch, this would be even better, but these are now hard to get - especially as linear.