Repeater emitter transistor: principle of operation

The history of transistors begins in the middle of the 20th century, when in 1956 three American physicists - D. Bardin, U. Brattein, V. Shockley, were awarded the Nobel Prize “For research of semiconductors and the discovery of the transistor effect”.

Radio engineering, starting work in his field, it is sometimes difficult to understand the electronic circuits and the purpose of one or another of its components. To do this, there are certain developments - already invented wiring diagrams for transistors and other elements with certain properties, from which it is possible to make various devices. One of these "bricks" in the building of electronic circuits is an emitter follower on a transistor.

Wiring diagram of transistors

There are three types of bipolar transistors - with a common base (OB), with a common emitter (OE) and a common collector (OK).

The most common connection (OE), because it gives a large gain voltage and current. One of the features of this connection is inverting the input voltage by 1800. The disadvantage of the connection is a small input (hundreds of ohms) and a large output (tens of ohms) resistance.

OE Transistor Circuit

When the input voltage is applied, the transistor opens and the current passes through the base to the emitter, and the collector current increases. The emitter current is summed from the base current and collector current: AndE= AndB+ AndTO

In the collector circuit, on a resistor,A voltage much higher than the input signal appears, which leads to an increase in the output voltage and, accordingly, of the current.

The inclusion of the transistor according to the scheme (ON) provides voltage gain and allows you to work with a wider frequency range than the scheme with (OE), so it is often used on antenna amplifiers. This scheme allows you to fully use the ability of the transistor to amplify the high frequency signal (frequency characteristics). The higher the frequency of the amplified signal, the lower the voltage gain. This cascade has a small input and output impedance.

The inclusion of the transistor with ON

The inclusion of the transistor with (OK) gives the current gain and is often used as an adapter between the high-resistance power supply and the low-impedance load.Also, this inclusion can be used in the coordination of various cascade circuits, it does not change the polarity of the input signal.

Turning on the transistor in the repeater emitter with OK

General concepts about repeater

The emitter follower is a current amplifier, in which the transistor is turned on according to the (OK) scheme. The voltage gain of the signal is practically equal to one, the emitter voltage is equal to the input signal, therefore the circuit is called the emitter follower. The principle of operation of the device will be discussed below.

Despite the fact that an emitter repeater has a voltage transfer ratio of one, it can be classified as an amplifier, since it gives a gain with respect to current, and hence with respect to power: AndE= (β +1) x AndBwhere andE- emitter current, AndB- base current.

With a small resistance of the power source, the collector of the transistor is connected to the common bus, and the resistor, from which the output voltage is removed, is connected to the emitter circuit. Connecting the input and output to external circuits by using capacitors C1and C2. With a small increase in voltage, the increase in current reaches its peak in the short circuit mode of the terminals at the output.

Operating principle

The load of the repeater cascade circuit is a resistor on the emitter PE.The input signal is fed through the first capacitor C1and the removal of the output signal occurs through the second capacitor C2.

The emitter voltage follower has a very small input and a large output impedance. With alternating current, when a half-wave of positive alternating voltage passes through a n-type transistor, it opens more and increases the current, while with a negative half-wave, vice versa. As a result, the output AC voltage has the same phase with the input and is the feedback voltage. The output voltage is directed towards the input and is connected in series, so the emitter follower uses consistent negative feedback. The output voltage is less than the input voltage by a small amount (the base-emitter voltage is about 0.6 V).

How to make a calculation scheme

Emitter follower transistor

The initial data to make the emitter follower calculation is collector current (AndTO) and supply voltage (YBX):

  • Emitter voltage (YE) must comply with:E= 0.5 x YBX(to provide maximum output for the output voltage).
  • Now you need to make a calculation of the resistance of the resistor on the emitter: PE= YE/ANDTO.
  • Calculation of resistance of a resistor divider is made: P1-R2(we select resistances so that the current on the divider is about 10 times less than the current on the base): AndD= 0.1 x AndTO/ β, where β is the current gain of the transistor. Resistance p1+ P2= YBX/ANDD.
  • Calculate the voltage base relative to the ground:B= YE+ 0,7.

Distinctive features

The emitter follower has an interesting feature - the collector current only depends on the load resistance and input voltage, and the parameters of the transistor do not play a significant role. Such schemes are considered to have 100% voltage feedback. You can not be afraid to burn the transistor, applying power to the base without a limiting resistor.

The work of the emitter follower is based on a high input impedance, which allows you to connect to it a signal source with a large complex impedance (for example, a pickup in the radio). Amplifier

Very often, an emitter repeater is used as a power amplifier in the output stages of amplifiers.The main task of such nodes is to transfer a certain power to the load. The most important parameter that is set in the amplifier’s power calculations is the power gain,distortion of signal transmission and efficiency (its increase is necessary due to the consumption of most of the power supply power by the output amplifier).Voltage gain is not the main parameter and usually approaches unity.

There are several ways to work such an amplifier stage, depending on the location of the operating point on the graph of characteristics and, accordingly, with different efficiency and characteristics of the output signal.

Modes of operation

In the considered cases of the emitter follower operation, the collector junction will be back shifted and the operation mode will depend on the emitter junction:

  1. In the first case, the emitter junction is shifted in such a way that the transistor stably does not go into saturation mode and the repeater works on the direct portion of the transfer characteristic graph (voltage UTOand yEare the same). The maximum output voltage is less than the input voltage.The efficiency is equal to the ratio of the power supplied to the load to the power from the power supply, and reaches a maximum (25%) at the highest amplitude of the output voltage. In order to avoid the output and input signal mismatch, the amplitude of the output voltage has to be reduced, as a result, the efficiency is also reduced. Low efficiency in this mode of operation of the repeater due to the independence of the current passing through the transistor, from the supply voltage and the power that is consumed from the power source is a constant value. In the absence of an input signal, the power dissipated by the transistor is greatest. Therefore, in this mode, the emitter follower is not used as a power amplifier, but rather as a transmitter of a slightly distorted signal.
  2. Another operating mode of the amplifier stage, in which the displacement of the emitter junction brings the operating point of the transistor to the boundary of the locking region. If we take the voltage of the emitter (E= 0) and the input signal is not received, the emitter junction is reverse biased and the transistor is in the closed state. As a result, reduced power consumption.When passing from the power source of the positive half-wave, the transistor is unlocked (the emitter junction opens), and the negative one blocks it (there is no output signal). The second case of the amplifier stage solves the problem with an increase in the efficiency of the amplifier, because there is no current on the transistor if there is no supply voltage. But there is a drawback - a strong distortion of the output signal.

Push-pull circuit

The push-pull emitter follower allows current amplification in the positive and negative ranges. To get a bipolar output signal, you can use a complementary emitter follower. In principle, the push-pull circuit is two repeaters, each of which amplifies the signal in a positive or negative half-wave. The circuit consists of two types of bipolar transistors (with pnp and pnp - transitions).

Push pull emitter follower

The principle of operation of the complementary scheme

When the input power is absent, both transistors are turned off, due to the absence of voltage at the emitter junction. With the passage of the half-wave of positive polarity, the discovery of the pnp transistor occurs, similarlythe passage of the negative half-wave causes the opening of the pnp - transistor.

A powerful emitter follower has an efficiency calculation (K = Pi / 4 x YOUT/ UTO) whereout- amplitude of the output signal; HaveTO- voltage at the collector junction.

From the formula it is clear that K increases with increasing amplitude.OUTand becomes maximum when YOUT= YTO(K = Pi / 4 = 0.785).

This shows that the emitter follower on the complementary scheme has a significantly higher efficiency than a conventional repeater.

The property of this scheme is large (transient) nonlinear distortion. They manifest themselves to a greater extent than the lower input voltage (YBX).

Calculation of the push-pull amplifier

Since we need an emitter follower for gain in power, the initial data to make the calculation of an emitter follower will be: load resistance (PH), the load power (PH). To reduce the mismatch of the output and input signal, the supply voltage should be higher by 5 V from the amplitude of the output voltage.

Formulas for calculating the amplifier stage:

  • Output voltage:OUT= square root (2PHRH).
  • Power supply voltage: AtBX= YE+ 5.
  • Output Current: AndE= YE/RH.
  • Power taken from the power source: P++ P-= 2 / Pi × YE/RH× yTO.
  • The maximum power dissipation on each of the transistors: P1= P2= YTO2/ Pi2RH.

Reduced Output Voltage Distortion

The push-pull emitter follower, the principle of operation of which is described above, can be further improved by reducing the transient distortion of the output signal in its circuit.

To reduce the voltage distortion at the output of the cascade can be applied to the base of the voltage transistors, bias the output characteristic.

Diodes or transistors are used to bias, which feed a signal to the bases of the working transistors of the repeater.

Diode Circuit

Complementary emitter follower with diodes

On the emitter transitions of transistors T1and t2an offset appears due to the diodes D1and D2connected between the bases of transistors. When the input voltage is zero, the transistors are active. When the voltage polarity is positive, the transistor T2locked, and with a negative voltage polarity locked the transistor T1. When the input signal is zero, one of the transistors is active, so the diode circuit gives an output signal characteristic that is very close to linear.Instead of diodes, transistors with shunted collector junction can be used.

Power amplifier with additional emitter repeaters

Another scheme that reduces the distortion of the output signal, the input of which includes two transistors.

Complementary emitter follower with input transistors

In this circuit, two repeaters on the transistor are placed at the input, which create a voltage offset for the emitter transitions of two output transistors. A significant advantage of this inclusion will be increased resistance at the input of the cascade. The emitter currents of the input and base currents of the output transistors, set the first two resistors.The second two resistors are included in the feedback circuit for output transistors.

This connection option is a single-gain buffer amplifier.

Composite transistors

Now transistors are produced as a separate cascade of two transistors in one package (Darlington circuit). They are used in chips in amplifiers on discrete components. Replacing a conventional transistor with an integral increases the input and decreases the output impedance of the circuit.

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