what is a circuit breaker,parallel circuit and open circuit?Knowledge of electrical circuits

It consists of two transistors with the same characteristics to form a symmetrical circuit, in the absence of an input signal, each tube is in the cutoff state, the quiescent current is almost zero, only when there is a signal input tube conduction, this state is known as class

B operating state. When the input signal is a sinusoidal waveform, VT1 conducts and VT2 cuts off during the positive half-cycle, and VT2 conducts and VT1 cuts off during the negative half-cycle. The alternating currents of the two tubes are synthesized in the output

transformer so that a pure sine wave is obtained on the load. This form of alternating two tubes is called push-pull circuit.

The widely used transformerless Class B push-pull amplifier, referred to as an OTL circuit, is a very high performance power amplifier. For ease of illustration, an OTL circuit with an input transformer and no output transformer is shown below.

Circuits that amplify DC signals or signals that change very slowly are called DC amplifier circuits or DC amplifiers. These amplifiers are commonly used in measurement and control.

DC amplifiers cannot be RC-coupled or transformer-coupled, only direct-coupled. The figure below shows a two-stage direct-coupled amplifier. The direct coupling method will bring about the mutual restraint of the front and rear stages, the circuit in the emitter of VT2 add resistor R E in order to increase the emitter potential of the rear stage to solve the front and rear stage of the restraint.

Another more important issue for DC amplifiers is zero drift. Zero drift refers to the slow change in static potential caused by the unstable operating point of the amplifier in the absence of an input signal, which is amplified stage by stage, resulting in a spurious signal at the output. The more amplifier stages, the more serious zero drift. Therefore, this kind of double-tube direct-coupled amplifier can only be used in less demanding occasions.

The solution to zero drift is to use a differential amplifier, the diagram below shows a widely used emitter coupled differential amplifier. It uses a dual power supply where VT1 and VT2 have the same characteristics, both resistors have the same value and R E has negative feedback. This is actually a bridge circuit, with the two R Cs and the two tubes being the four bridge arms, and the output voltage, V 0, is taken out of the bridge on the diagonal. When there is no input signal, the bridge is balanced and the output is zero because RC1=RC2 and the two tubes have the same characteristics. Since it is connected as a bridge, the zero drift is also small. Differential amplifiers have good stability and are therefore widely used.

An integrated operational amplifier is a device that puts a multistage DC amplifier on an integrated chip and can perform various functions by connecting a small number of components externally. It is called operational amplifier because it was used in analog computers as an adder and multiplier in the early days.

A circuit that automatically converts DC energy into an AC signal of a certain amplitude and frequency without the need for an external signal is called an oscillating circuit or oscillator. This phenomenon is also called self-excited oscillation. Alternatively, a circuit capable of generating an AC signal is called an oscillator circuit.

An oscillator must consist of three parts: an amplifier, a positive feedback circuit and a frequency selection network. The amplifier amplifies the input signal added at the input of the oscillator so that the output signal maintains a constant value. The positive feedback circuit ensures that the feedback signals provided to the oscillator input are in phase so that the oscillation can be maintained. The frequency selector network allows only a specific frequency f0 to pass through so that the oscillator produces a single frequency output.

The ability of the oscillator to oscillate and maintain a stable output is determined by the following two conditions; one is the feedback voltage Uf and the input voltage Ui to be equal, which is the amplitude balance condition. The second is that Uf and Ui must be in phase, which is the phase balance condition, that is to say, it must be guaranteed to be positive feedback. In general, the amplitude balance condition is often easy to do, so in determining whether an oscillator circuit can oscillate, the main thing is to see whether its phase balance condition is valid.

Oscillator can be divided into ultra-low frequency (20 Hz or less), low frequency (20 Hz ~ 200 kHz), high frequency (200 kHz ~ 30 MHz) and ultra-high frequency (10 MHz ~ 350 MHz) and so on. According to the oscillation waveform can be divided into sine wave oscillation and non-sinusoidal oscillation two categories.

Sine wave oscillators can be divided into LC oscillators, RC oscillators and quartz crystal oscillators according to the components used in the frequency selection network. Quartz crystal oscillator has a very high frequency stability, only in demanding occasions. In general household appliances, a large number of LC oscillators and RC oscillators are used.

The frequency selection network of an LC oscillator is an LC resonant circuit. They all oscillate at relatively high frequencies and there are three common circuits.
1) Transformer feedback LC oscillator

Figure (a) shows a transformer feedback LC oscillator circuit. The transistor VT is a common-emitter amplifier. The primary of transformer T is a frequency selective LC resonant circuit, and the secondary of transformer T provides a positive feedback signal to the amplifier input. When the power supply is turned on, there is a weak transient current in the LC loop, but only a current with the same frequency as the resonant frequency of the loop, f 0, can generate a high voltage at both ends of the loop, which is coupled through the primary stage of the transformer, L1 and L2, and then returned to the base of the transistor, V. The voltage is then fed back to the base of the transistor, VT, which is a common emitter amplifier. As can be seen in figure (b), as long as the connection is not incorrect, this feedback signal voltage is in phase with the input signal voltage, i.e., it is a positive feedback. Therefore, the oscillation of the circuit is rapidly intensified and finally stabilized.
Transformer feedback LC oscillator circuit is characterized by: wide frequency range, easy to vibrate, but the frequency stability is not high. Its oscillation frequency is: f 0 =1/2π LC. Commonly used to generate tens of kilohertz to tens of megahertz sine wave signals.
2) Inductive three-point oscillation circuit

Figure (a) is another commonly used inductive three-point oscillator circuit. In the figure, inductors L1, L2 and capacitor C form a resonant circuit for frequency selection. A feedback voltage is taken from L2 and added to the base of transistor VT. As can be seen in Fig. (b), the input and feedback voltages of the transistor are in phase, which satisfies the phase balance condition, and thus the circuit is able to oscillate. Since the three poles of the transistor are connected to the three points of the inductor, the circuit is called an inductive three-point oscillator.

Inductive three-point oscillation circuit is characterized by a wide frequency range, easy to oscillate, but the output contains more high modulation waveforms, the waveform is poor. Its oscillation frequency is: f 0 =1/2π LC, where L = L1 + L2 + 2M. It is commonly used to generate sine wave signals below tens of megahertz.
 

3) Capacitive three-point oscillation circuit

6.2 RC Oscillators