Bipolar transistor biasing

The basic circuits of an amplifier stage are named after the electrode, which is located on a clearly defined electrical potential. Those electrode is the one that is input and output circuit common. In the case of a bipolar transistor with its three electrodes emitter, collector and base arise as the emitter circuit, the collector circuit and the base circuit. Because of their properties, the collector circuit is usually called an emitter follower. The transistor basic circuits differ essentially in their electrical properties and therefore in use.

  • 3.1 dimensioning of the components
  • 3.2 Features
  • 3.3 Areas of application
  • 4.1 properties
  • 4.2 Areas of application


The arrangement contained in a device such as an audio amplifier to a variety of electronic building blocks can be (at least conceptually) divided into dozens of basic circuits described herein. The overall function resulting from the combination and interaction of the individual basic circuits.

The following are the basic circuits are described with bipolar detail. Instead of bipolar transistors of the analog circuits described may also be realized with field effect transistors ( FET ) or electron tubes. Although the properties of the corresponding circuits are not identical, are similar, however, due to the same underlying principles in their behavior. The corresponding FET circuits are common source, drain circuit / source follower and the gate circuit called as the cathode base tube circuits to analog circuit follower cathode / anode base circuit and hot grating base circuit.

The circuits are usually shown in the following figure in the top row, to clarify the respective common electrode. However, the operation will be apparent when the circuitry contained within bold according to the bottom row.

The above method for determining the respective basic circuit is not always strictly satisfied, so that a further criterion must be applied: The name of the basic circuit is carried out according to the electrode of the transistor, which is located at the common reference potential of the input and output.

Emitter circuit

The emitter circuit is based on the basic function of the bipolar transistor: A current flowing into the base signal alternating current causes a larger order the AC gain of alternating current in the collector forth.

The figure 2 shows an amplifier stage for AC voltage in the emitter circuit with capacitive bypassed emitter resistor. With the resistors and the operating point is set. In addition, you can see here an operating point stabilization (see section below ) by DC negative feedback by means of resistance. The capacitors determine the lower cutoff frequency of the circuit. They are so large that they can be regarded as the useful signal to be amplified AC as a short circuit. sets the emitter AC coupled to ground. and block the DC components of input and output. The base current to control the larger the AC gain collector -emitter current.

The input resistance is small and corresponds to the parallel circuit, and the base -emitter resistance. If omitted, it increases the input resistance, because then instead of the resistor goes into the account. The output resistance of the parallel circuit of the load resistor and the collector -emitter resistance (which is typically very large as compared with ). The voltage gain is in the absence of the ratio of to, otherwise it is of type transistor and the temperature. The emitter current is equal to the collector current plus the base current.

A disadvantage is the reduction of the switching frequency by the Miller effect. This can be avoided by a two cascode transistors.

We made a similar basic circuit with field effect transistors is called a common source; the corresponding basic circuit with electron tubes called cathode base circuit.

Dimensioning of the components

The voltage should be at about 0.6 V, the voltage at 1.2 V. Thus, should be so large that the voltage at the collector is quite exactly half as large as the operating voltage, since both half-waves will reach its maximum value. The circuit in Figure 2 has no AC negative feedback and therefore distorts the signal. This can be significantly improved by a small resistance of about 100 Ω in series. However, this also decreases the gain.


  • Inverting
  • High current gain
  • High voltage gain
  • Power gain about 100-1000, about voltage gain × current gain
  • Input resistance: 500 Ω -2 kOhm
  • Output resistance: 50-100 kOhm or approximately equal to the load resistor R3
  • Low-distortion amplification only for very small input voltages: when C3 Available < 0.001V, otherwise depending on the ratio

Areas of application

The emitter arrangement is used in many areas of electronics, for example, in small-signal amplifiers, and electronic switches. It is by far the most frequently encountered of the three basic circuits.

Stabilization of the operating point

The nature of the stabilization of the operating point is in principle independent of the basic transistor circuit. The following are the stabilization circuits:

  • Stabilization by emitter resistor or DC negative feedback ( see Figure 2)
  • DC voltage feedback ( see Figure 3)

Collector circuit ( emitter follower )

The serving power source for the signal to have no resistance (possibly a capacitor in parallel), therefore, the collector at a constant voltage level. In the circuit a small base-emitter current to flow, and controls a large collector -emitter current. This is determined by the load resistance; is at that a voltage with the input voltage and the base -emitter voltage of approximately 0.7 V.

Therefore, the output voltage at the emitter approximately follows the input voltage, which is why it is also called an emitter follower circuit. Since the current will be reduced by the load resistance at the input by a factor of the current gain, the input impedance of an emitter follower circuit is very high, the voltage gain is about 1, which makes the circuit to an impedance converter.

We made a similar basic circuit with field effect transistors is called a drain or source follower circuit; the corresponding basic circuit with electron tubes called cathode follower or anode base circuit.

Dimensioning of the components

The voltage on should be quite exactly half as large as the operating voltage, because both half-waves then can reach its maximum value. This is achieved if and are equal in size.


  • Non - Inverting
  • Voltage gain of almost 1
  • High current gain
  • Power gain almost equal to the current gain
  • Large input resistance: 3 kOhm 1 M (load resistance × current gain )
  • Output resistance is small: 0.5-30 Ω
  • Low-distortion transmission for input voltages up to the supply voltage

Areas of application

Impedance converter, eg for crystal pickups and piezo sound sensor in condenser and electret microphones, as a precursor of the Darlington circuit (in this case the load is the base of the output stage ) and many audio amplifier output stages.

Basic circuit

It corresponds to the emitter circuit, however, the base is connected to ground or a constant voltage and the emitter current must also flow through the source. This makes the current gain to the first, the input resistance is very small, since the entire load current and the base current needs to be applied from the source. The output resistance and the voltage gain which respectively correspond to the emitter circuit.

We made a similar basic circuit with field effect transistors is called a gate circuit; the corresponding basic circuit with electron tubes called grid base circuit.


  • Non - Inverting
  • Current gain of slightly less than 1
  • High voltage gain
  • Power gain about 1000 ⇒ voltage gain

Areas of application

  • RF stages
  • RF oscillators from about 50 MHz


Through combinations of the basic circuits, the following circuits arise:

  • Parallel connection: several transistors are connected in parallel, with bipolar transistors, however, each requires its own emitter resistor to ensure current sharing (not required for MOSFET and IGBT)
  • Cascade; Series connection of several transistors in the emitter circuit, the reverse voltages add up, each transistor requires a separate, electrically isolated base drive
  • Cascode: An emitter circuit ( below) with overlying base circuit results in a cascode amplifier in which the input resistance is low and the output resistance is very high. This circuit has particularly small reactions and is therefore suitable for high frequency applications.
  • Transistor - transistor logic inverter: the basic circuit with a subsequent emitter circuit.
  • Darlington circuit. Two transistors in the collector circuit in succession; the base of the second is the load of the first, they share the voltage between base and emitter 1 2 Darlington circuit can be viewed as a single transistor with high current gain, it also integrated Darlington circuits, called Darlington transistors, made.
  • Thyristor circuit, multivibrator: Two emitter circuits with feedback.
  • Schmitt Trigger: two transistors collector-connected, but with a common emitter resistor.

In the output stages of the TTL technology, two transistors are operated in a half-bridge arrangement, the lower emitter, in the collector circuit of the upper.

The current mirror, the second transistor operates in a common emitter circuit, the first, the voltage at the base of the second ready so that its collector current equal to the input current; Use as a controllable current source.

The differential input, such as an operational amplifier the two inputs of each acts as an emitter circuit ( inverting) to the next level associated with it, however, as a result of the collector circuit and the basic circuit of the other output.