In
electronics
The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...
, a common-base (also known as grounded-base)
amplifier
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost the v ...
is one of three basic single-stage
bipolar junction transistor
A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipolar ...
(BJT) amplifier topologies, typically used as a
current buffer or
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
amplifier
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost the v ...
.
In this circuit the emitter terminal of the transistor serves as the input, the collector as the output, and the base is connected to ground, or "common", hence its name. The analogous
field-effect transistor
The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs (JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs contro ...
circuit is the
common-gate
In electronics, a common-gate amplifier is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a current buffer or voltage amplifier. In this circuit, the source terminal of the transistor serve ...
amplifier.
Applications
This arrangement is not very common in low-frequency discrete circuits, where it is usually employed for amplifiers that require an unusually low
input impedance The input impedance of an electrical network is the measure of the opposition to current ( impedance), both static ( resistance) and dynamic ( reactance), into the load network that is ''external'' to the electrical source. The input admittance (the ...
, for example to act as a
preamplifier
A preamplifier, also known as a preamp, is an electronic amplifier that converts a weak electrical signal into an output signal strong enough to be noise-tolerant and strong enough for further processing, or for sending to a power amplifier an ...
for moving-coil
microphone
A microphone, colloquially called a mic or mike (), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public ...
s.
However, it is popular in integrated circuits and in high-frequency amplifiers, for example for
VHF
Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter.
Frequencies immediately below VHF ...
and
UHF
Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter (on ...
, because its input capacitance does not suffer from the
Miller effect In electronics, the Miller effect accounts for the increase in the equivalent input capacitance of an inverting voltage amplifier due to amplification of the effect of capacitance between the input and output terminals. The virtually increased input ...
, which degrades the bandwidth of the
common-emitter
In electronics, a common-emitter amplifier is one of three basic single-stage bipolar-junction-transistor (BJT) amplifier topologies, typically used as a voltage amplifier. It offers high current gain (typically 200), medium input resistance a ...
configuration, and because of the relatively high isolation between the input and output. This high isolation means that there is little feedback from the output back to the input, leading to high stability.
This configuration is also useful as a current buffer, since it has a current gain of approximately
unity
Unity may refer to:
Buildings
* Unity Building, Oregon, Illinois, US; a historic building
* Unity Building (Chicago), Illinois, US; a skyscraper
* Unity Buildings, Liverpool, UK; two buildings in England
* Unity Chapel, Wyoming, Wisconsin, US; a h ...
(see formulae below). Often a common base is used in this manner, preceded by a common-emitter stage. The combination of these two form the
cascode
The cascode is a two-stage amplifier that consists of a common-emitter stage feeding into a common-base stage.
Compared to a single amplifier stage, this combination may have one or more of the following characteristics: higher input–output ...
configuration, which possesses several of the benefits of each configuration, such as high input impedance and isolation.
Low-frequency characteristics
At low frequencies and under
small-signal
Small-signal modeling is a common analysis technique in electronics engineering used to approximate the behavior of electronic circuits containing nonlinear devices with linear equations. It is applicable to electronic circuits in which the AC si ...
conditions, the circuit in Figure 1 can be represented by that in Figure 2, where the
hybrid-pi model The hybrid-pi model is a popular circuit model used for analyzing the small signal behavior of bipolar junction and field effect transistors. Sometimes it is also called Giacoletto model because it was introduced by L.J. Giacoletto in 1969. The m ...
for the BJT has been employed. The input signal is represented by a
Thévenin voltage source ''v''
s with a series resistance ''R''
s and the load is a resistor ''R''
L.
This circuit can be used to derive the following characteristics of the common base amplifier.
:Note: Parallel lines (, , ) indicate
components in parallel.
In general, the overall voltage/current gain may be substantially less than the open/short-circuit gains listed above (depending on the source and load resistances) due to the
loading effect
In electronics, a voltage divider (also known as a potential divider) is a passive circuit, passive linear circuit that produces an output voltage (''V''out) that is a fraction of its input voltage (''V''in). Voltage division is the result of dis ...
.
Active loads
For voltage amplification, the range of allowed output voltage swing in this amplifier is tied to voltage gain when a resistor load ''R
C'' is employed, as in Figure 1. That is, large voltage gain requires large ''R
C'', and that in turn implies a large DC voltage drop across ''R
C''. For a given supply voltage, the larger this drop, the smaller the transistor ''V
CB'' and the less output swing is allowed before saturation of the transistor occurs, with resultant distortion of the output signal. To avoid this situation, an
active load
An active load or dynamic load is a electronic component, component or a electronic circuit, circuit that functions as a current-stable nonlinear resistor.
Circuit design
In circuit design, an active load is a circuit component made up of ''active ...
can be used, for example, a
current mirror
A current mirror is a circuit designed to copy a current through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. The current being "copied" can be, and ...
. If this choice is made, the value of ''R
C'' in the table above is replaced by the small-signal output resistance of the active load, which is generally at least as large as the ''r
O'' of the active transistor in Figure 1. On the other hand, the DC voltage drop across the active load has a fixed low value (the
compliance voltage of the active load), much less than the DC voltage drop incurred for comparable gain using a resistor ''R
C''. That is, an active load imposes less restriction on the output voltage swing. Notice that active load or not, large AC gain still is coupled to large AC output resistance, which leads to poor voltage division at the output except for large loads ''R''
L ≫ ''R''
out.
For use as a current buffer, gain is not affected by ''R
C'', but output resistance is. Because of the current division at the output, it is desirable to have an output resistance for the buffer much larger than the load ''R''
L being driven, so large signal currents can be delivered to a load. If a resistor ''R
C'' is used, as in Figure 1, a large output resistance is coupled to a large ''R
C'', again limiting the signal swing at the output. (Even though current is delivered to the load, usually a large current signal into the load implies a large voltage swing across the load as well.) An active load provides high AC output resistance with much less serious impact upon the amplitude of output signal swing.
Overview of characteristics
Several example applications are described in detail below. A brief overview follows.
*The amplifier input impedance ''R''
in looking into the emitter node is very low, given approximately by
::
:where ''V
T'' is the
thermal voltage
The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a ideal gas, gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas ...
, and ''I
E'' is the DC emitter current.
:For example, for ''V
T'' = 26 mV and ''I
E'' = 10 mA, rather typical values, ''R''
in = 2.6 Ω. If ''I
E'' is reduced to increase ''R''
in, there are other consequences like lower transconductance, higher output resistance and lower β that also must be considered. A practical solution to this low-input-impedance problem is to place a common-emitter stage at the input to form a
cascode
The cascode is a two-stage amplifier that consists of a common-emitter stage feeding into a common-base stage.
Compared to a single amplifier stage, this combination may have one or more of the following characteristics: higher input–output ...
amplifier.
*Because the input impedance is so low, most signal sources have larger source impedance than the common-base amplifier ''R''
in. The consequence is that the source delivers a ''current'' to the input rather than a voltage, even if it is a voltage source. (According to
Norton's theorem
In direct-current circuit theory, Norton's theorem, also called the Mayer–Norton theorem, is a simplification that can be applied to networks made of linear time-invariant resistances, voltage sources, and current sources. At a pair of term ...
, this current is approximately ''i''
in = ''v''
S / ''R''
S). If the output signal also is a current, the amplifier is a current buffer and delivers the same current as is input. If the output is taken as a voltage, the amplifier is a
transresistance Transconductance (for transfer conductance), also infrequently called mutual conductance, is the electrical characteristic relating the current through the output of a device to the voltage across the input of a device. Conductance is the reciproca ...
amplifier and delivers a voltage dependent on the load impedance, for example ''v''
out = ''i''
in ''R''
L for a resistor load ''R''
L much smaller in value than the amplifier output resistance ''R''
out. That is, the voltage gain in this case (explained in more detail below) is
::
:Note that for source impedances such that ''R''
S ≫ ''r
E'' the output impedance approaches ''R''
out = ''R
C'' , ,
m'' (''rπ'' ">, ''RS'') ''rO''
*For the special case of very low-impedance sources, the common-base amplifier does work as a voltage amplifier, one of the examples discussed below. In this case (explained in more detail below), when ''R''
S ≪ ''r
E'' and ''R''
L ≪ ''R''
out, the voltage gain becomes
::
:where ''g
m'' = ''I
C'' / ''V
T'' is the transconductance. Notice that for low source impedance, ''R''
out = ''r
O'' , , ''R
C''.
*The inclusion of ''r
O'' in the hybrid-pi model predicts reverse transmission from the amplifiers output to its input, that is the amplifier is bilateral. One consequence of this is that the input/output
impedance is affected by the load/source termination impedance, hence, for example, the output resistance ''R''
out may vary over the range ''r
O'' , , ''R
C'' ≤ ''R''
out ≤ (β + 1) ''r
O'' , , ''R
C'', depending on the source resistance ''R''
S. The amplifier can be approximated as unilateral when neglect of ''r
O'' is accurate (valid for low gains and low to moderate load resistances), simplifying the analysis. This approximation often is made in discrete designs, but may be less accurate in RF circuits, and in integrated-circuit designs, where active loads normally are used.
Voltage amplifier
For the case when the common-base circuit is used as a voltage amplifier, the circuit is shown in Figure 2.
The output resistance is large, at least ''R
C'' , , ''r
O'', the value which arises with low source impedance (''R''
S ≪ ''r
E''). A large output resistance is undesirable in a voltage amplifier, as it leads to poor
voltage division
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to mo ...
at the output. Nonetheless, the voltage gain is appreciable even for small loads: according to the table, with ''R
S = r
E'' the gain is ''A
v = g
m R
L / 2''. For larger source impedances, the gain is determined by the resistor ratio ''R
L / R
S'', and not by the transistor properties, which can be an advantage where insensitivity to temperature or transistor variations is important.
An alternative to the use of the hybrid-pi model for these calculations is a general technique based upon
two-port network
A two-port network (a kind of four-terminal network or quadripole) is an electrical network ( circuit) or device with two ''pairs'' of terminals to connect to external circuits. Two terminals constitute a port if the currents applied to them sati ...
s. For example, in an application like this one where voltage is the output, a g-equivalent two-port could be selected for simplicity, as it uses a voltage amplifier in the output port.
For ''R
S'' values in the vicinity of ''r
E'' the amplifier is transitional between voltage amplifier and current buffer. For ''R
S'' ≫ ''r
E'' the driver representation as a
Thévenin source should be replaced by representation with a
Norton source. The common base circuit stops behaving like a voltage amplifier and behaves like a current follower, as discussed next.
Current follower
Figure 3 shows the common base amplifier used as a current follower. The circuit signal is provided by an AC
Norton source (current ''I
S'', Norton resistance ''R
S'') at the input, and the circuit has a resistor load ''R
L'' at the output.
As mentioned earlier, this amplifier is bilateral as a consequence of the output resistance ''r
O'', which connects the output to the input. In this case the output resistance is large even in the worst case (it is at least ''r
O , , R
C'' and can become ''(β + 1) r
O , , R
C'' for large ''R
S''). Large output resistance is a desirable attribute of a current source because favorable
current division
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
sends most of the current to the load. The current gain is very nearly unity as long as ''R
S ≫ r
E''.
An alternative analysis technique is based upon
two-port network
A two-port network (a kind of four-terminal network or quadripole) is an electrical network ( circuit) or device with two ''pairs'' of terminals to connect to external circuits. Two terminals constitute a port if the currents applied to them sati ...
s. For example, in an application like this one where current is the output, an h-equivalent two-port is selected because it uses a current amplifier in the output port.
See also
*
Common collector
In electronics, a common collector amplifier (also known as an emitter follower) is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a voltage buffer.
In this circuit the base terminal o ...
*
Common emitter
In electronics, a common-emitter amplifier is one of three basic single-stage bipolar-junction-transistor (BJT) amplifier topologies, typically used as a voltage amplifier. It offers high current gain (typically 200), medium input resistance a ...
*
Common gate
In electronics, a common-gate amplifier is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a current buffer or voltage amplifier. In this circuit, the source terminal of the transistor serves ...
*
Common drain
In electronics, a common-drain amplifier, also known as a source follower, is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a voltage buffer. In this circuit (NMOS) the gate terminal of the t ...
*
Common source
In electronics, a common-source amplifier is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a voltage or transconductance amplifier. The easiest way to tell if a FET is common source, comm ...
*
Differential amplifier
A differential amplifier is a type of electronic amplifier that amplifies the difference between two input voltages but suppresses any voltage common to the two inputs. It is an analog circuit with two inputs V_\text^- and V_\text^+ and one outpu ...
*
Hybrid-pi model The hybrid-pi model is a popular circuit model used for analyzing the small signal behavior of bipolar junction and field effect transistors. Sometimes it is also called Giacoletto model because it was introduced by L.J. Giacoletto in 1969. The m ...
*
Two-port network
A two-port network (a kind of four-terminal network or quadripole) is an electrical network ( circuit) or device with two ''pairs'' of terminals to connect to external circuits. Two terminals constitute a port if the currents applied to them sati ...
References
External links
ECE 327: Transistor Basics— Gives example common base circuit (i.e., current source) with explanation.
{{DEFAULTSORT:Common Base
Single-stage transistor amplifiers