Biasing in electronics is the method of establishing predetermined voltages or currents at various points of an electronic circuit for the purpose of establishing proper operating conditions in electronic components. Many electronic devices such as transistors and vacuum tubes, whose function is processing time‐varying (AC) signals also require a steady (DC) current or voltage to operate correctly; this is called bias. The AC signal applied to them is superposed on this DC bias current or voltage. The operating point of a device, also known as bias point, quiescent point, or Q‐point, is the steady‐state voltage or current at a specified terminal of an active device (a transistor or vacuum tube) with no input signal applied
Before an ac signal is fed to a transistor, it is necessary to set up an operating point or the quiescent(Q) point of operation. Generally this Q point is set at the middle of the DC load line. Once the Q point is set, then the incoming ac signals can produce fluctuations above and below this Q point as shown in
The need for biasing &transistor can also be explained as follows;
For a transistor to remain operating in the linear region, the emitter diode must remain forward biased and the collector diode must remain reverse biased as long as the amplifier is amplifying. In other words, the amplitude variations in current and voltage of the input signal must not drive the transistor either into saturation or cut off.
Stable Q point
A set Q point of a transistor amplifier may shift due to Increased temperature and transistor P value changes. Therefore, the objective of good biasing is to limit this shifting of the Q point or to achieve a stable Q point
Stabilization of operating point is needed due to following reasons
In practice the operating point varies shifts due to drift in temperature e.t.c. As temperature increases Ico, βdc, Vbe gets affected. The reverse saturation current almost doubles for every 10 degree rise in collector junction temperature. The base to emitter voltage decreases by 2.5 milli volts for every one degree rise in temperature. Hence the operating point should be stabilized against the variations in temperature so that it remains stable. To 286 achieve this biasing circuits are introduced
Types of transistor biasing
There are several ways to bias a transistor for operation. This means, there are several ways of setting up a Q point near the middle of the dc load line. Important biasing arrangements used with transistors are:
1 BASE BIAS: Fig shows one type of biasing of transistor known as base‐bias. As shown, usually, the collector voltage supply itself is used for the base voltage instead of a separate supply
2 EMlTER BIAS or emitter feedback bias:
This type of biasing compensates for the variations in ßdc and keeps the Q point fairly stable.
if ßdc increases, the collector current increases. This in turn increases the voltage at the emitter. This increased emitter voltage decreases the voltage across the base‐emitter junction and therefore ,the base current reduces. This reduced base current results in less collector current, which partially offsets the increase in IC due to increased ßdc Emitter bias is also referred to as emitter feedback bias.
This is because an output quantity, i.e., the collector current, produces a change in an input quantity i.e., the base current. The term feedback means a portion of the output is given back to the input. In emitter bias, the emitter resistor is the feedback element because it is common to both the output and input circuits
3 VOLTAGE‐DIVIDER METHOD
Also known as the universal bias because, this is the most widely used type of biasing in linear circuits
This type of biasing is known as voltage divider bias because of the voltage divider formed by resistors R, and R,. The voltage drop across R, should be such that it forward biases the emitter diode.
Gain of transistor
- Common Base Configuration – has Voltage Gain but no Current Gain.
- Common Emitter Configuration – has both Current and Voltage Gain.
- Common Collector Configuration – has Current Gain but no Voltage Gain
Where: Ic/Ie is the current gain, alpha ( α ) and RL/Rin is the resistance gain.
The common base circuit is generally only used in single stage amplifier circuits such as microphone pre‐amplifier or radio frequency ( Rf ) amplifiers due to its very good high frequency response
In Common emitter Gain
In this type of configuration, the current flowing out of the transistor must be equal to the currents flowing into the transistor as the emitter current is given as Ie = Ic + Ib.
As the load resistance ( RL ) is connected in series with the collector, the current gain of the common emitter transistor configuration is quite large as it is the ratio of Ic/Ib. A transistors current gain is given the Greek symbol of Beta, ( β ).
As the emitter current for a common emitter configuration is defined as Ie = Ic + Ib, the ratio of Ic/Ie is called Alpha, given the Greek symbol of α. Note: that the value of Alpha will always be less than unity.
Since the electrical relationship between these three currents, Ib, Ic and Ie is determined by the physical construction of the transistor itself, any small change in the base current ( Ib ), will result in a much larger change in the collector current ( Ic ).
Then, small changes in current flowing in the base will thus control the current in the emitter‐collector circuit. Typically, Beta has a value between 20 and 200 for most general purpose transistors. So if a transistor has a Beta value of say 100, then one electron will flow from the base terminal for every 100 electrons flowing between the emitter‐collector terminals.
By combining the expressions for both Alpha, α and Beta, β the mathematical relationship between these parameters and therefore the current gain of the transistor can be given as:
Where: “Ic” is the current flowing into the collector terminal, “Ib” is the current flowing into the base terminal and “Ie” is the current flowing out of the emitter terminal.
Then to summarize a little. This type of bipolar transistor configuration has a greater input impedance, current and power gain than that of the common base configuration but its voltage gain is much lower. The common emitter configuration is an inverting amplifier circuit. This means that the resulting output signal is 180o “out‐of‐phase” with the input voltage signal
Common collector gain
As the emitter current is the combination of the collector AND the base current combined, the load· resistance in this type of transistor configuration also has both the collector current and the input current of the base flowing through it. Then the current gain of the circuit is given as
This type of bipolar transistor configuration is a non‐inverting circuit in that the signal voltages of Vin and Vout are “in‐phase”. It has a voltage gain that is always less than “1” (unity). The load resistance of the common collector transistor receives both the base and collector currents giving a large current gain (as with the common emitter configuration) therefore, providing good current amplification with very little voltage gain.
Note: That for the Power Gain you can also divide the power obtained at the output with the power obtained at the input. Also when calculating the gain of an amplifier, the subscripts v, i and p are used to denote the type of signal gain being used.
The power Gain or power level of the amplifier can also be expressed in Decibels, (dB). The Bel (B) is a logarithmic unit (base 10) of measurement that has no units. Since the Bel is too large a unit of measure, it is prefixed with deci making it Decibels instead with one decibel being one tenth (1/10th) of a Bel. To calculate the gain of the amplifier in Decibels or dB, we can use the following expressions.
- Voltage Gain in dB: av = 20 log Av
- Current Gain in dB: ai = 20 log Ai
- Power Gain in dB: ap = 10 log Ap
Note that the DC power gain of an amplifier is equal to ten times the common log of the output to input ratio, where as voltage and current gains are 20 times the common log of the ratio. Note however, that 20dB is not twice as much power as 10dB because of the log scale.
