PART8Basic Electrical Circuit

Experiment Purpose

• 1.Understands amplifier and multistage amplifier.
• 2.Understands characteristic of cascade amplifier.
• 3.Understand constant voltage circuit, relay and touch control switch.

Experiment 1 :Amplifier

Theory

Amplification

As in fig.8-2, when small electrical signal is applied to the input terminal, big electrical signal appears at the output terminal, and this is called the amplification and the device to execute this is called the amplifier.

This means that the signal itself does not gets bigger but the output of amplifier gets proportionally bigger to the input signal.

When the signal is applied to the input terminal of the amplifier, the purpose of amplifier is to amplify the signal as we desire. For example, if AC signal is applied, the output that is the same type as the input signal should be amplified at the output terminal, but in the actual situation, it is not. There are several reasons for this.

Experiment Process

As in fig.8-3, if AC signal is applied to the amplifier, the output that is the same type as the input signal should be amplified at the output terminal. However, at the output terminal, the waveform that is distorted as dotted line is amplified, and this is called distortion.

• The amplitude distortion is caused by incorrect bias operating point, excessive input signal, no straight characteristic curve, incorrect load value and so on, so the input signal should be applied to the operating point of A level or the bias or the load value should be chosen correctly.
• The frequency distortion is caused at the output by lots of harmonics occurred at the active element, and this phenomenon means the change of amplitude due to the circuit constant by the frequency. If possible, the active element which contains less harmonics should be used and the circuit constant proper to the frequency should be selected.
• The phase distortion is caused by the combination between the preference reaching to the input directly and that reaching later due to various causes. This is a big problem in the high frequency area but does not matter in the low frequency area, that is, in the sound equipment.
• The signal appears at the output side though it is not applied to the input, and this is called noise and there are various kinds of noises such as the noise caused by the element, mechanical noise, electrical noise and so on. There are various measured for noise removal, such as using circuit element that has correct value, or doing the complete shielding by reducing the capacity between parts. In case of parasitic oscillation, neutralizing circuit should be used.

Amplification Circuit of Transistor

There are C-B(common base) circuit, C-E(common emitter) circuit, and C-C(common collector) circuit.

- C-B(Common Base) Circuit

Common base circuit is the circuit that the base is commonly connected to both the input and output of the circuit. The base should be earthed and the input signal should be applied between the emitter and the base so the output is earned between the base and the collector.

In fig.8-4, the direction of the arrow is that of the current and I_E=I_C+I_B(A) is valid. Here, the power(bias) should be applied so that the current can flow in the arrow direction. Supply by comparing the direction of I_E and the polarity of V_EE and the direction of I_C and the polarity of V_CC.

The amplification factor α of this amplifier is

This amplifier has the lowest input impedance and highest output impedance, and its current amplification degree is nearly 1 and the voltage amplification degree is big. Also, its cutoff frequency is high so it is used frequently in the high frequency amplification circuit.

- C-E(Common Emitter) Circuit

This is the circuit that the emitter is earthed and the input voltage is applied to the base and the output is earned at the collector. To understand C-E circuit, the characteristic of the circuit between the input terminals or between the base emitters, or the circuit between the output terminals or between the collector emitters.

In fig.8-5, the direction of the arrow is that of the current and even though the composition of the transistor changes, the relationship between the currents are not different from that of C-B circuit.

That is, I_E `=I<sub>C`</sub> `+I_B(A) and I_C = alpha I_E(A)

The amplification factor β of this amplifier is

Ifαis indicated by β, it is α= β/(1+β)..

The characteristic of this amplifier is that its amplification degrees of current and voltage are over 1, and its input and output impedances are the middle of collector earthing and base earthing. The input and output impedance do not change much according to the signal source and load resistance, and the amplification factors of current and voltage are efficiently big so this is generally used in the amplification circuit.

- C-C(Common Collector) Circuit

Common collector circuit is usually called emitter follower, and it has load(output) resistance between the emitter and the earthing. The shape is similar to C-E circuit but the difference is that the collector is earthed. The direction of arrow in fig.8-6 indicates the direction of the current, and C-C circuit, contrary to C-B or C-E circuits, has big input impedance and small output impedance so is used in the impedance matching circuit, and its output characteristic is same as that of C-E circuit.

The characteristic of this circuit is that its input impedance is big and output impedance is small so it is proper to be combined with the circuit having low input impedance(impedance matching). Also, the voltage phase of input and output is same and the gain is below 1, the current phase of input and output is reversed and the gain is big. This is 100% negative feedback amplifier that is stable and whose distortion is the smallest.

Transistor Bias Circuit

Bias means the device that can be operated as an amplifier by applying DC voltage to the transistor amplifier and flowing DC current. The amplification of transistor is possible when proper DC voltage is supplied and DC current flows.

- Types of Bias

There are base bias, emitter bias, negative feedback bias and the combination bias of these kinds.

Generally, emitter bias or negative feedback bias are widely used, which solve the problem of base bias that the bias changes according to the temperature of surroundings or of the transistor itself.

Voltage Division Bias Circuit

Fig.8-7 is called voltage distribution bias circuit or current feedback bias circuit, which divides the power voltage of the circuit Vcc with R1 and R2 resistances and gets the bias voltage VBE by the voltage drop of R4 resistance connected to the emitter.

When the input signal is applied to the base, the change of base current occurs by changes of emitter current and collector current.

A_V=V_O/V_i , the current gain is A_i = I_O / I_i, and the power gain is A_P = A_O / A_i = P_O / P_i.

Experiment Process

1. Make a connection as the circuit in fig.8-8.

2. Turn on the switch of the circuit and measure the voltage V_BE, V_CE, V_EE of TR1 and record the result in the relevant column of table 8-1.

3. Measure the voltage between R3’s both ends and calculate the collector current I_C using the measured voltage and Ohm’s law.

4. Connect the function generator to the input terminal V_in and measure with the oscillator the output voltage of the collector when applying the voltage of 1kHz, 0.5Vpp and record the result in the relevant column of table 8-1. Calculate the voltage gain and record the result.

5. Measure the emitter output voltage with the oscilloscope and record the result in the relevant column of table 8-1. Calculate the voltage gain and record the result.

6. Draw the input voltage(V_in) waveform and output voltage(V_CE, V_EE) waveform in the relevant column of table 8-1 when the input voltage(>0.5Vpp) is increased in the range that the output waveform(V_CE) is not distorted and record the output voltage.

- Calculate the voltage amplification degree with measured input voltage and output voltages of collector and emitter.

tab1 print