12/5/24, 9:34 AM Common Emitter Amplifier and Transistor Amplifiers
Common Emitter Amplifier
Common Emitter
Amplifier
The most common amplifier configuration for an
NPN transistor is that of the Common Emitter
Amplifier circuit
Transistor amplifier’s amplify an AC input signals that alternates between some positive
value and a corresponding negative value. Then some way of “presetting” a common
emitter amplifier circuit configuration is required so that the transistor can operate
between these two maximum or peak values. This can be achieved using a process
known as Biasing.
Biasing is very important in amplifier design as it establishes the correct operating point
of the transistor amplifier ready to receive signals, thereby reducing any distortion to the
output signal.
Also, the use of a static or DC load line drawn onto the output characteristics curves of
an amplifier allows us to see all the possible operating points of the transistor from fully
“ON” to fully “OFF”, and to which the quiescent operating point or Q-point of the
amplifier can be found.
The aim of any small signal amplifier is to amplify all of the input signal with the
minimum amount of distortion possible to the output signal, in other words, the output
signal must be an exact reproduction of the input signal but only bigger (amplified).
To obtain low distortion when used as an amplifier the operating quiescent point needs
to be correctly selected. This is in fact the DC operating point of the amplifier and its
position may be established at any point along the load line by a suitable biasing
arrangement.
The best possible position for this Q-point is as close to the center position of the load
line as reasonably possible, thereby producing a Class A type amplifier operation,
ie. Vce = 1/2Vcc. Consider the Common Emitter Amplifier circuit shown below.
The Common Emitter Amplifier Circuit
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12/5/24, 9:34 AM Common Emitter Amplifier and Transistor Amplifiers
The single stage common emitter amplifier circuit shown above uses what is commonly
called “Voltage Divider Biasing”. This type of biasing arrangement uses two resistors as
a potential divider network across the supply with their center point supplying the
required Base bias voltage to the transistor. Voltage divider biasing is commonly used in
the design of bipolar transistor amplifier circuits.
This method of biasing the transistor greatly reduces the
effects of varying Beta, ( β ) by holding the Base bias at a
constant steady voltage level allowing for best stability.
The quiescent Base voltage (Vb) is determined by the
potential divider network formed by the two
resistors, R1, R2 and the power supply voltage Vcc as
shown with the current flowing through both resistors.
Then the total resistance RT will be equal to R1 + R2 giving
the current as i = Vcc/RT. The voltage level generated at the
junction of resistors R1 and R2 holds the Base voltage (Vb)
constant at a value below the supply voltage.
The potential divider network used in the common emitter amplifier circuit divides the
supply voltage in proportion to the resistance. This bias reference voltage can be easily
calculated using the simple voltage divider formula below:
Transistor Bias Voltage
As the same supply voltage, (Vcc) also determines the maximum Collector
current, Ic when the transistor is switched fully “ON” (saturation), Vce = 0. The Base
current Ib for the transistor is found from the Collector current, Ic and the DC current
gain Beta, β of the transistor.
Beta Value
A transistor’s Beta value, sometimes referred to as hFE on datasheets, defines the
transistor’s forward current gain in the common emitter configuration. Beta is an
electrical parameter built into the transistor during manufacture. Beta (hFE) has no units
as it is a fixed ratio of the two currents, Ic and Ib so a small change in the Base current
will cause a large change in the Collector current.
One final point about Beta. Transistors of the same type and part number will have
large variations in their Beta value. For example, the BC107 NPN Bipolar transistor can
have a DC current gain Beta value of between 110 and 450 (data sheet value).
That is, one BC107 device may have a Beta value of 110, while another one may have
a Beta value of 450, but they are both sold as BC107 npn transistors. The reason for
this is that the value of Beta ( β ) is an inherent characteristic of the transistor’s
construction and not of its operation.
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12/5/24, 9:34 AM Common Emitter Amplifier and Transistor Amplifiers
As the Base/Emitter junction is forward-biased, the Emitter voltage, Ve will be one
junction voltage drop different to the Base voltage. If the voltage across the Emitter
resistor is known then the Emitter current, Ie can be easily calculated using Ohm’s Law.
The Collector current, Ic can be approximated, since it is almost the same value as the
Emitter current.
Common Emitter Amplifier Example No1
A common emitter amplifier circuit has a load resistance, RL of 1.2kΩ and a supply
voltage of 12v. Calculate the maximum Collector current (Ic) flowing through the load
resistor when the transistor is switched fully “ON” (saturation), assume Vce = 0.
Also find the value of the Emitter resistor, RE if it has a voltage drop of 1v across it.
Calculate the values of all the other circuit resistors assuming a standard NPN silicon
transistor.
This then establishes point “A” on the Collector current vertical axis of the
characteristics curves and occurs when Vce = 0. When the transistor is switched fully
“OFF”, there is no voltage drop across either resistor RE or RL as no current is flowing
through them. Then the voltage drop across the transistor, Vce is equal to the supply
voltage, Vcc. This establishes point “B” on the horizontal axis of the characteristics
curves.
Generally, the quiescent Q-point of the amplifier is with zero input signal applied to the
Base, so the Collector sits about half-way along the load line between zero volts and
the supply voltage, (Vcc/2). Therefore, the Collector current at the Q-point of the
amplifier will be given as:
This static DC load line produces a straight line equation whose slope is given
as: -1/(RL + RE) and that it crosses the vertical Ic axis at a point equal to Vcc/(RL + RE).
The actual position of the Q-point on the DC load line is determined by the mean value
of Ib.
As the Collector current, Ic of the transistor is also equal to the DC gain of the transistor
(Beta), times the Base current (β*Ib), if we assume a Beta (β) value for the transistor of
say 100, (one hundred is a reasonable average value for low power signal transistor)
the Base current Ib flowing into the transistor will be given as:
Instead of using a separate Base bias supply, it is usual to provide the Base Bias
Voltage from the main supply rail (Vcc) through a dropping resistor, R1.
Resistors, R1 and R2 can now be chosen to give a suitable quiescent Base current of
45.8μA or 46μA rounded off to the nearest integer. The current flowing through the
potential divider circuit has to be large compared to the actual Base current, Ib, so that
the voltage divider network is not loaded by the Base current flow.
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12/5/24, 9:34 AM Common Emitter Amplifier and Transistor Amplifiers
A general rule of thumb is a value of at least 10 times Ib flowing through the resistor R2.
Transistor Base/Emitter voltage, Vbe is fixed at 0.7V (silicon transistor) then this gives
the value of R2 as:
If the current flowing through resistor R2 is 10 times the value of the Base current, then
the current flowing through resistor R1 in the divider network must be 11 times the value
of the Base current. That is: IR2 + Ib.
Thus the voltage across resistor R1 is equal to Vcc – 1.7v (VRE + 0.7 for silicon
transistor) which is equal to 10.3V, therefore R1 can be calculated as:
The value of the Emitter resistor, RE can be easily calculated using Ohm’s Law. The
current flowing through RE is a combination of the Base current, Ib and the Collector
current Ic and is given as:
Resistor, RE is connected between the transistor’s Emitter terminal and ground, and we
said previously that there is a voltage drop of 1 volt across it. Thus the value of the
Emitter resistor, RE is calculated as:
So, for our example above, the preferred values of the resistors chosen to give a
tolerance of 5% (E24) are:
Then, our original Common Emitter Amplifier circuit above can be rewritten to include
the values of the components that we have just calculated above.
Completed Common Emitter Circuit
Amplifier Coupling Capacitors
In Common Emitter Amplifier circuits, capacitors C1 and C2 are used as Coupling
Capacitors to separate the AC signals from the DC biasing voltage. This ensures that
the bias condition set up for the circuit to operate correctly is not affected by any
additional amplifier stages, as the capacitors will only pass AC signals and block any
DC component. The output AC signal is then superimposed on the biasing of the
following stages. Also a bypass capacitor, CE is included in the Emitter leg circuit.
This capacitor is effectively an open circuit component for DC biasing conditions, which
means that the biasing currents and voltages are not affected by the addition of the
capacitor maintaining a good Q-point stability.
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12/5/24, 9:34 AM Common Emitter Amplifier and Transistor Amplifiers
However, this parallel connected bypass capacitor effectively becomes a short circuit to
the Emitter resistor at high frequency signals due to its reactance. Thus only RL plus a
very small internal resistance acts as its load increasing voltage gain to its maximum.
Generally, the value of the bypass capacitor, CE is chosen to provide a reactance of at
most, 1/10th the value of RE at the lowest operating signal frequency.
Output Characteristics Curves
Ok, so far so good. We can now construct a series of curves that show the Collector
current, Ic against the Collector/Emitter voltage, Vce with different values of Base
current, Ib for our simple common emitter amplifier circuit.
These curves are known as the “Output Characteristic Curves” and are used to show
how the transistor will operate over its dynamic range. A static or DC load line is drawn
onto the curves for the load resistor RL of 1.2kΩ to show all the transistor’s possible
operating points.
When the transistor is switched “OFF”, Vce equals the supply voltage Vcc and this is
point “B” on the line. Likewise, when the transistor is fully “ON” and saturated the
Collector current is determined by the load resistor, RL and this is point “A” on the line.
We calculated before from the DC gain of the transistor that the Base current required
for the mean position of the transistor was 45.8μA and this is marked as point Q on the
load line which represents the Quiescent point or Q-point of the amplifier. We could
quite easily make life easy for ourselves and round off this value to 50μA exactly,
without any effect to the operating point.
Output Characteristics Curves
Point Q on the load line gives us the Base current Q-point of Ib = 45.8μA or 46μA. We
need to find the maximum and minimum peak swings of Base current that will result in a
proportional change to the Collector current, Ic without any distortion to the output
signal.
As the load line cuts through the different Base current values on the DC characteristics
curves we can find the peak swings of Base current that are equally spaced along the
load line. These values are marked as points “N” and “M” on the line, giving a minimum
and a maximum Base current of 20μA and 80μA respectively.
These points, “N” and “M” can be anywhere along the load line that we choose as long
as they are equally spaced from Q. This then gives us a theoretical maximum input
signal to the Base terminal of 60μA peak-to-peak, (30μA peak) without producing any
distortion to the output signal.
Any input signal giving a Base current greater than this value will drive the transistor to
go beyond point “N” and into its “cut-off” region or beyond point “M” and into its
Saturation region thereby resulting in distortion to the output signal in the form of
“clipping”.
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Using points “N” and “M” as an example, the instantaneous values of Collector current
and corresponding values of Collector-emitter voltage can be projected from the load
line. It can be seen that the Collector-emitter voltage is in anti-phase (–180o) with the
collector current.
As the Base current Ib changes in a positive direction from 50μA to 80μA, the Collector-
emitter voltage, which is also the output voltage decreases from its steady state value
of 5.8 volts to 2.0 volts.
Then a single stage Common Emitter Amplifier is also an “Inverting Amplifier” as an
increase in Base voltage causes a decrease in Vout and a decrease in Base voltage
produces an increase in Vout. In other words, the output signal is 180o out-of-phase
with the input signal.
Common Emitter Voltage Gain
The Voltage Gain of the common emitter amplifier is equal to the ratio of the change in
the input voltage to the change in the amplifier’s output voltage.
Then ΔVL is Vout and ΔVB is Vin. But voltage gain is also equal to the ratio of the signal
resistance in the Collector to the signal resistance in the Emitter and is given as:
We mentioned earlier that as the ac signal frequency increases the bypass
capacitor, CE starts to short out the Emitter resistor due to its reactance. Then at high
frequencies RE = 0, making the gain infinite.
However, the bipolar transistor has a small internal resistance built
into its Emitter region called r’e. The transistor’s semiconductor
material offers an internal resistance to the flow of current through it
and is generally represented by a small resistor symbol shown inside
the main transistor symbol.
Transistor data sheets tell us that for a small signal bipolar transistor this internal
resistance is the product of 25mV ÷ Ie (25mV being the internal volt drop across the
Emitter junction layer), then for our common Emitter amplifier circuit above this
resistance value will be equal to:
This internal Emitter leg resistance will be in series with the external Emitter
resistor, RE, then the equation for the transistor’s actual gain will be modified to include
this internal resistance so will be:
At low frequency signals the total resistance in the Emitter leg is equal to RE + r’e. At
high frequency, the bypass capacitor shorts out the Emitter resistor leaving only the
internal resistance r’e in the Emitter leg resulting in a high gain.
Then for our common emitter amplifier circuit above, the gain of the circuit at both low
and high signal frequencies is given as:
Amplifier Gain at Low Frequencies
Amplifier Gain at High Frequencies
Thus at very low input signal frequencies, the reactance of the capacitor (XC) is high so
the external emitter resistance, RE has an effect on voltage gain lowering it to, in this
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example, 5.32. However, when the input signal frequency is very high, the reactance of
the capacitor shorts out RE (RE = 0) so the amplifier’s voltage gain increases to, in this
example, 218.
One final point, the voltage gain is dependent only on the values of the Collector
resistor, RL and the Emitter resistance, (RE + r’e) it is not affected by the current gain
Beta, β (hFE) of the transistor.
So, for our simple example above we can now summarise all the values we have
calculated for our common emitter amplifier circuit and these are:
Minimum Mean Maximum
Base Current 20μA 50μA 80μA
Collector Current 2.0mA 4.8mA 7.7mA
Output Voltage Swing 2.0V 5.8V 9.3V
Amplifier Gain -5.32 -218
Common Emitter Amplifier Summary
Then to summarise. The Common Emitter Amplifier circuit has a resistor in its
Collector circuit. The current flowing through this resistor produces the voltage output of
the amplifier. The value of this resistor is chosen so that at the amplifier’s quiescent
operating point, Q-point this output voltage lies half way along the its load line.
The Base of the transistor used in a common emitter amplifier is biased using two
resistors as a potential divider network. This type of biasing arrangement is commonly
used in the design of bipolar transistor amplifier circuits and greatly reduces the effects
of varying Beta, ( β ) by holding the Base bias at a constant steady voltage. This type of
biasing produces the greatest stability.
A resistor can be included in the emitter leg in which case the voltage gain becomes -
RL/RE. If there is no external Emitter resistance, the voltage gain of the amplifier is not
infinite as there is a very small internal resistance, r’e in the Emitter leg. The value of
this internal resistance is equal to 25mV/IE
In the next tutorial about bipolar transistor amplifiers we will look at the Junction Field
Effect Amplifier commonly called the JFET Amplifier. Like the transistor, the JFET is
used in a single stage amplifier circuit making it easier to understand.
There are several different kinds of field effect transistor that we could use but the
easiest to understand is the junction field effect transistor, or JFET which has a very
high input impedance making it ideal for amplifier circuits.
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