The Common Base Amplifier is another type of bipolar junction transistor, (BJT) configuration where the base terminal of the transistor is a common terminal to both the input and output signals, hence its name common base (CB). The common base configuration is less common as an amplifier than compared to the more popular common emitter, (CE) or common collector, (CC) configurations but is still used due to its unique input/output characteristics.
For the common base configuration to operate as an amplifier, the input signal is applied to the emitter terminal and the output is taken from the collector terminal. Thus the emitter current is also the input current, and the collector current is also the output current, but as the transistor is a three layer, two pn-junction device, it must be correctly biased for it to work as a common base amplifier. That is the base-emitter junction is forward-biased.
Consider the basic common base amplifier configuration below.
Common Base Amplifier using an NPN Transistor
Then we can see from the basic common base configuration that the input variables relate to the emitter current IE and the base-emitter voltage, VBE, while the output variables relate to the collector current IC and the collector-base voltage, VCB.
Since the emitter current, IE is also the input current, any changes to the input current will create a corresponding change in the collector current, IC. For a common base amplifier configuration, current gain, Ai is given as iOUT/iIN which itself is determined by the formula IC/IE. The current gain for a CB configuration is called Alpha, ( α ).
In a BJT amplifier the emitter current is always greater than the collector current as IE = IB + IC, the current gain (α) of the amplifier must therefore be less than one (unity) as IC is always less than IE by the value of IB. Thus the CB amplifier attenuates the current, with typical values of alpha ranging from between 0.980 to 0.995.
The electrical relationship between the three transistor currents can be shown to give the expressions for alpha, α and Beta, β as shown.
Common Base Amplifier Current Gain
Therefore if the Beta value of a standard bipolar junction transistor is 100, then the value of Alpha would be given as: 100/101 = 0.99.
Common Base Amplifier Voltage Gain
Since the common base amplifier can not operate as a current amplifier (Ai ≅ 1), it must therefore have the ability to operate as a voltage amplifier. The voltage gain for the common base amplifier is the ratio of VOUT/VIN, that is the collector voltage VC to the emitter voltage VE. In other words, VOUT = VC and VIN = VE.
as the output voltage VOUT is developed across the collector resistance, RC, the output voltage must therefore be a function of IC as from Ohms Law, VRC = IC*RC. So any change in IE will have a corresponding change in IC.
Then we can say for a common base amplifier configuration that:
As IC/IE is alpha, we can present the amplifiers voltage gain as:
Therefore the voltage gain is more or less equal to ratio of the collector resistance to the emitter resistance. However, there is a single pn-diode junction within a bipolar junction transistor between the base and emitter terminals giving rise to what is called the transistors dynamic emitter resistance, r’e.
For AC input signals the emitter diode junction has an effective small-signal resistance given by: r’e = 25mV/IE, where the 25mV is the thermal voltage of the pn-junction and IE is the emitter current. So as the current flowing through the emitter increases, the emitter resistance will decrease by a proportional amount.
Some of the input current flows through this internal base-emitter junction resistance to the base as well as through the externally connected emitter resistor, RE. For small-signal analysis these two resistances are connected in parallel with each other.
Since the value of r’e is very small, and RE is generally much larger, usually in the kilohms (kΩ) range, the magnitude of the amplifiers voltage gain changes dynamically with different levels of emitter current.
Thus if RE ≫ r’e then the true voltage gain of the common base amplifier will be:
Because the current gain is approximately equal to one as IC ≅ IE, then the voltage gain equation simplifies to just:
So if for example, 1mA of current is flowing through the emitter-base junction, its dynamic impedance would be 25mV/1mA = 25Ω. The volt gain, AV for a collector load resistance of 10kΩ would be: 10,000/25 = 400, and the more current which flows through the junction, the lower becomes its dynamic resistance and the higher the voltage gain.
Likewise, the higher the value of load resistance the greater the amplifiers voltage gain. However, a practical common base amplifier circuit would be unlikely to use a load resistor greater than about 20kΩ with typical values of voltage gain range from about 100 to 2000 depending on the value of RC. Note that the amplifiers power gain is about the same as its voltage gain.
As the voltage gain of the common base amplifier is dependant on the ratio of these two resistive values, it therefore follows that there is no phase inversion between the emitter and the collector. Thus the input and output waveforms are “in-phase” with each other showing that the common base amplifier is non-inverting amplifier configuration.
Common Base Amplifier Resistance Gain
One of the interesting characteristics of the common base amplifier circuit is the ratio of its input and output impedances giving rise to what is known as the amplifiers Resistance Gain, the fundamental property which makes amplification possible. We have seen above that the input is connected to the emitter and the output taken from the collector.
Between the input and ground terminal there are two possible parallel resistive paths. One through the emitter resistance, RE to ground and the other through r’e and the base terminal to ground. Thus we can say looking into the emitter with the base grounded that: ZIN = RE||r’e.
But as the dynamic emitter resistance, r’e is very small compared to RE (r’e≪RE), the internal dynamic emitter resistance, r’e dominates the equation resulting in a low input impedance approximately equal to r’e
So for the common base configuration the input impedance is very low and depending on the value of the source impedance, RS connected to emitter terminal, input impedance values can range from between 10Ω and 200Ω. The low input impedance of the common base amplifier circuit is one of the main reason for its limited applications as a single stage amplifier.
The output impedance of the CB amplifier however, can be high depending on the collector resistance used to control the voltage gain and the connected external load resistance, RL. If a load resistance is connected across the amplifiers output terminal, it is effectively connected in parallel with the collector resistance, then ZOUT = RC||RL.
But if the externally connected load resistance, RL is very large compared to the collector resistance RC, then RC will dominate the parallel equation, resulting in a moderate output impedance ZOUT, becoming approximately equal to RC. Then for a common base configuration, its output impedance looking back into the collector terminal would be: ZOUT = RC.
As the output impedance of the amplifier looking back into the collector terminal can potentially be very large, the common base circuit operates almost like an ideal current source taking the input current from the low input impedance side and sending the current to the high output impedance side. Thus the common base transistor configuration is also referred to as a: current buffer or current follower configuration, and the opposite of the common-collector (CC) configuration which is referred to as a voltage follower.
Common Base Amplifier Summary
We have seen here in this tutorial about the Common Base Amplifier that it has a current gain (alpha) of approximately one (unity), but also a voltage gain that can be very high with typical values ranging from 100 to over 2000 depending on the value of the collector load resistor RL used.
We have also seen that the input impedance of the amplifier circuit is very low, but the output impedance can be very high. We also said that the common base amplifier does not invert the input signal as it is a non-inverting amplifier configuration.
Due to its input-output impedance characteristics, the common base amplifier arrangement is extremely useful in audio and radio frequency applications as a current buffer to match a low-impedance source to a high-impedance load or as a single stage amplifier as part of a cascoded or multi-stage configuration where one amplifier stage is used to drive another.
