Voltage Divider Equation
However, with this type of polarity arrangement, the voltage divider is not loaded by the base current because the mains is too small, so if there is any change in the Vcc supply voltage, then the voltage level at the base will also change proportionally. Next, base polarization of transistors or some kind of voltage stabilization of the Q-point is required.
Transmitter Resistance Stabilization
The polarization of the amplifiers can be stabilized by placing a single resistance in the transistor emitter circuit as shown. This resistance is known as Transmitter Resistance, RE. The addition of this emitter resistance means that the transistor emitter terminal is no longer grounded or has zero volt potential, but instead has a small potential given by the Ohm Act equation: AND = IE x RE. Where: IE is the actual emitter current. Now if the supply voltage increases Vcc, the transistor collector current for a certain load resistance increases in Ic. If the collector current increases, the corresponding emitter current should also increase, which will lead to increased voltage drop throughout THE. This action causes a proportional increase in base voltage because VB = AND + VBE. Since the base voltage is held steady by the dividing resistors R1 and R2, the DC voltage at the base is reduced by a proportional amount compared to the emitter Vbe, thereby reducing the base current drive and preventing further increase of collector current. A similar action occurs if the supply voltage and collector current try to depreciate. In other words, the addition of this absorbent terminal resistance helps to control the base polarization of transistors using negative feedback, which rejects any attempt changes in the collector current with a contrasting change in the base pre-voltage, so that the circuit tends to stabilize at a fixed level. . In addition, since part of the feed is reduced through RE, its value must be as small as possible in order to develop the maximum possible voltage through the load resistance, RL and therefore the output. However, its value cannot be too small, or once again the instability of the circuit is damaged. Then the current passing through the emitter resistance is calculated as follows:
Transmitter Resistance Current
As a general rule, the voltage drop in this emitter resistance is usually taken as follows: VB – VBE or one-tenth of the feed voltage value (1/10) Vcc. A common figure for the emitter resistance voltage, whichever is lower, is from 1 to 2 volts. The value of transmitter resistance can also be found in the gain, as the RE now equals ac voltage gain: RL /RE
Transmitter Resistance Example
A common emitter riser has the following characteristics, β = 100, Vcc = 30V and RL = 1kΩ. Calculate the resistance if the amplifier circuit uses an emitter resistance to improve stability. The amplifier calm current, ICQ is given as follows: the voltage drop in emitter resistance is usually between 1 and 2 volts, so let's assume that ve has a voltage drop of 1.5 volts. In this case, the Transmitter Resistance value required for the amplifier circuit: 100Ω and the final common emitter circuit are given as follows:
Common Emitter Amplifier
If necessary, the gain of the amplifier stage can also be found and given as follows:
Transmitter Bypass Condenser
In the basic serial feedback circuit above, emitter resistance, RE performs two functions: DC negative feedback for stable polarity and AC negative feedback for signal transmission and voltage gain feature. However, since the emitter resistance is a feedback resistance, it will also reduce amplifier gain due to IE fluctuations in the emitter current due to the AC input signal. To overcome this problem, a capacitor called "Emitter Bypass Capacitor" is attached CE to the emitter resistance as shown. This bypass capacitor bypasses the signal currents into the soil (hence the name) at a specified cutting frequency, causing the amplifier's frequency response to be interrupted at εc. Since it is a capacitor, it appears as an open circuit for DC bias, and therefore, polar currents and voltages are not affected by the addition of the bypass capacitor. Over the operating frequency range of amplifiers, capacitor colorant, XC, will be extremely high at low frequencies and will produce a negative feedback effect, reducing amplifier gain. The CE value of this bypass capacitor is usually selected to provide a capacitive reactance of up to a tenth (1/10) of the value of the emitter resistance RE at the lowest cutting frequency point. Next, the lowest signal frequency to be raised is assumed to be 100 Hz. The CE value of the bypass capacitor is calculated as follows: Then, for our simple common emitter riser above the value of the emitter bypass capacitor, which is connected in parallel with the emitter resistance: 160μF
Split Transmitter Amplifier
While the addition of the bypass capacitor helps to control amplifier gain by countering the effects of CE, beta uncertainty, one of the main drawbacks (β) is that the capacitor reactance at high frequencies is so low that it effectively shorts out. The result is that at high frequencies the reassurance of the capacitor allows very little AC feedback control, since re becomes a short circuit, which means that the AC voltage gain of the transistor greatly increases by dragging the amplifier to saturation. An easy way to control amplifier gain across the entire operating frequency range is to divide the emitter resistance into two parts, as shown.
Split Transmitter Resistors
The resistance in the transmitter leg is divided into two parts: RE1 and RE2 form a voltage dividing network within the emitter leg with the bypass capacitor, which is connected in parallel along the lower resistance. The upper resistance RE1 is the same value as the previous one but is not bypassed by the capacitor, so it should be taken into account when calculating signal parameters. The lower resistance RE2 is connected parallel to the capacitor and is considered zero ohm when calculating signal parameters as it shorts out at high frequencies. The advantage here is that we can control the AC gain of the amplifier in the entire range of input frequencies. The total value of emitter resistance in DC is equal to RE1 + RE2, while at higher AC frequencies, the emitter resistance is only: RE1, as in the original non-bypass circuit above. So what's the resistance value, RE2. This will depend on the DC voltage gain required at the lower frequency breakpoint. We have already said that the gain of the above circuit is equal: RL/RE calculated as 10 (1kΩ/100Ω) for our common emitter circuit above. But now the gain in DC will be equal to: RL / (RE1 + RE2) Therefore, if we choose a DC gain of 1 (one) value of the value of the emitter resistance, re2 is given as follows:
Split emitter Resistance, RE2
Then re1 = 100Ω and RE2 = 900Ω for 1 (one) DC gain. Note that ac earnings will be the same at 10. Next, a split emitter riser has voltage gain and input impedance values somewhere between those of a fully bypassed emitter riser and an unpasteurized emitter riser, depending on the operating frequency.
The current amplification parameter of a transistor can vary significantly from one device with the same type and part number to another due to β, production tolerances, as well as changes in feed voltage and operating temperature. Next, for a common emitter class A amplifier circuit, it is necessary to use a pre-redevelopment circuit that will stabilize the DC collector current, the working Q-point that makes the IC independent of beta. The effect of β on the value of the emitter current can be reduced by adding a Transmitter Resistance, RE to the emitter leg to provide stabilization. The voltage drop in this emitter resistance is usually given from 1 to 2 volts. Transmitter resistance can be fully bypassed with a CE-suitable bypass capacitor that connects in parallel with transmitter resistance to achieve a higher AC gain, or partially bypassed using a split emitter voltage divider network that reduces DC gain and distortion. The value of this capacitor is determined by the capacitive reacx (XC) at the lowest signal frequency.