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Types of bias circuit for Class A amplifiers

The following discussion treats five common biasing circuits used with Class A bipolar transistor amplifiers: 1)Fixed bias 2)Collector-to-base bias 3)Fixed bias with emitter resistor 4)Voltage divider bias 5)Emitter bias

This form of biasing is also called base bias. In the example image on the right, the single power source (for example, a battery) is used for both collector and base of a transistor, although separate batteries can also be used. In the given circuit, Vcc = IBRB + Vbe Therefore, IB = (Vcc - Vbe)/RB For a given transistor, Vbe does not vary significantly during use. As Vcc is of fixed value, on selection of RB, the base current IB is fixed. Therefore this type is called fixed bias type of circuit. Also for given circuit, Vcc = ICRC + Vce Therefore, Vce = Vcc - ICRC The common-emitter current gain of a transistor is an important parameter in circuit design, and is specified on the data sheet for a particular transistor. It is denoted as β on this page. Because IC = βIB we can obtain IC as well. In this manner, operating point given as (Vce,IC) can be set for given transistor. Merits: It is simple to shift the operating point anywhere in the active region by merely changing the base resistor (RB). A very small number of components are required. Demerits: The collector current does not remain constant with variation in temperature or power supply voltage. Therefore the operating point is unstable. Changes in Vbe will change IB and thus cause RE to change. This in turn will alter the gain of the stage. When the transistor is replaced with another one, considerable change in the value of β can be expected. Due to this change the operating point will shift. For small-signal transistors (e.g., not power transistors) with relatively high values of β (i.e., between 100 and 200), this configuration will be prone to thermal runaway. In particular, the stability factor, which is a measure of the change in collector current with changes in reverse saturation current, is approximately β+1. To ensure absolute stability of the amplifier, a stability factor of less than 25 is preferred, and so small-signal transistors have large stability factors.[citation needed] Usage: Due to the above inherent drawbacks, fixed bias is rarely used in linear circuits (i.e., those circuits which use the transistor as a current source). Instead, it is often used in circuits where transistor is used as a switch. However, one application of fixed bias is to achieve crude automatic gain control in the transistor by feeding the base resistor from a DC signal derived from the AC output of a later stage. Collector-to-base bias[edit source | editbeta]
 * Fixed bias (Base bias)

Collector-to-base bias This configuration employs negative feedback to prevent thermal runaway and stabilize the operating point. In this form of biasing, the base resistor R_{\text{B}} is connected to the collector instead of connecting it to the DC source V_{\text{cc}}. So any thermal runaway will induce a voltage drop across the R_{\text{C}} resistor that will throttle the transistor's base current. From Kirchhoff's voltage law, the voltage V_{\text{R}_{\text{b}}} across the base resistor R_{\text{b}} is V_{\text{R}_{\text{b}}} = V_{\text{cc}} \, - \, \mathord{\overbrace{(I_{\text{c}} + I_{\text{b}}) R_{\text{c}}}^{\text{Voltage drop across } R_{\text{c}}}} \, - \, \mathord{\overbrace{V_{\text{be}}}^{\text{Voltage at base}}}. By the Ebers–Moll model, I_{\text{c}} = \beta I_{\text{b}}, and so V_{\text{R}_{\text{b}}} = V_{\text{cc}} - (\overbrace{\beta I_{\text{b}}}^{I_{\text{c}}} + I_{\text{b}}) R_{\text{c}} - V_{\text{be}} = V_{\text{cc}} - I_{\text{b}} (\beta + 1) R_{\text{c}} -  V_{\text{be}}. From Ohm's law, the base current I_{\text{b}} = V_{\text{R}_{\text{b}}} / R_{\text{b}}, and so \overbrace{I_{\text{b}} R_{\text{b}}}^{V_{\text{R}_{\text{b}}}} = V_{\text{cc}} - I_{\text{b}} (\beta + 1) R_{\text{c}} - V_{\text{be}}. Hence, the base current I_{\text{b}} is I_{\text{b}} = \frac{ V_{\text{cc}} - V_{\text{be}} }{ R_{\text{b}} + ( \beta + 1 ) R_{\text{c}} } If V_{\text{be}} is held constant and temperature increases, then the collector current I_{\text{c}} increases. However, a larger I_{\text{c}} causes the voltage drop across resistor R_{\text{c}} to increase, which in turn reduces the voltage V_{\text{R}_{\text{b}}} across the base resistor R_{\text{b}}. A lower base-resistor voltage drop reduces the base current I_{\text{b}}, which results in less collector current I_{\text{c}}. Because an increase in collector current with temperature is opposed, the operating point is kept stable. Merits: Circuit stabilizes the operating point against variations in temperature and β (i.e. replacement of transistor) Demerits: In this circuit, to keep I_{\text{c}} independent of \beta, the following condition must be met: I_{\text{c}} = \beta I_{\text{b}} = \frac { \beta (V_{\text{cc}} - V_{\text{be}})}{R_{\text{b}} + R_{\text{c}} + \beta R_{\text{c}}} \approx \frac{(V_{\text{cc}} - V_{\text{be}})}{R_{\text{c}}} which is the case when \beta R_{\text{c}} \gg R_{\text{b}}. As \beta-value is fixed (and generally unknown) for a given transistor, this relation can be satisfied either by keeping R_{\text{c}} fairly large or making R_{\text{b}} very low. If R_{\text{c}} is large, a high V_{\text{cc}} is necessary, which increases cost as well as precautions necessary while handling. If R_{\text{b}} is low, the reverse bias of the collector–base region is small, which limits the range of collector voltage swing that leaves the transistor in active mode. The resistor R_{\text{b}} causes an AC feedback, reducing the voltage gain of the amplifier. This undesirable effect is a trade-off for greater Q-point stability. Usage: The feedback also decreases the input impedance of the amplifier as seen from the base, which can be advantageous. Due to the gain reduction from feedback, this biasing form is used only when the trade-off for stability is warranted. Fixed bias with emitter resistor[edit source | editbeta]

Fixed bias with emitter resistor The fixed bias circuit is modified by attaching an external resistor to the emitter. This resistor introduces negative feedback that stabilizes the Q-point. From Kirchhoff's voltage law, the voltage across the base resistor is VRb = VCC - IeRe - Vbe. From Ohm's law, the base current is Ib = VRb / Rb. The way feedback controls the bias point is as follows. If Vbe is held constant and temperature increases, emitter current increases. However, a larger Ie increases the emitter voltage Ve = IeRe, which in turn reduces the voltage VRb across the base resistor. A lower base-resistor voltage drop reduces the base current, which results in less collector current because Ic = β IB. Collector current and emitter current are related by Ic = α Ie with α ≈ 1, so increase in emitter current with temperature is opposed, and operating point is kept stable. Similarly, if the transistor is replaced by another, there may be a change in IC (corresponding to change in β-value, for example). By similar process as above, the change is negated and operating point kept stable. For the given circuit, IB = (VCC - Vbe)/(RB + (β+1)RE). Merits: The circuit has the tendency to stabilize operating point against changes in temperature and β-value. Demerits: In this circuit, to keep IC independent of β the following condition must be met: I_C = \beta I_B = \frac { \beta (V_{CC} - V_{be})}{R_B+ ( \beta+1) R_E} \approx \frac {(V_{CC} - V_{be})}{R_E} which is approximately the case if ( β + 1 )RE >> RB. As β-value is fixed for a given transistor, this relation can be satisfied either by keeping RE very large, or making RB very low. If RE is of large value, high VCC is necessary. This increases cost as well as precautions necessary while handling. If RB is low, a separate low voltage supply should be used in the base circuit. Using two supplies of different voltages is impractical. In addition to the above, RE causes ac feedback which reduces the voltage gain of the amplifier. Usage: The feedback also increases the input impedance of the amplifier when seen from the base, which can be advantageous. Due to the above disadvantages, this type of biasing circuit is used only with careful consideration of the trade-offs involved. Collector-Stabilized Biasing Voltage divider biasing[edit source | editbeta]

Voltage divider bias The voltage divider is formed using external resistors R1 and R2. The voltage across R2 forward biases the emitter junction. By proper selection of resistors R1 and R2, the operating point of the transistor can be made independent of β. In this circuit, the voltage divider holds the base voltage fixed independent of base current provided the divider current is large compared to the base current. However, even with a fixed base voltage, collector current varies with temperature (for example) so an emitter resistor is added to stabilize the Q-point, similar to the above circuits with emitter resistor. In this circuit the base voltage is given by: V_B = \ voltage across R_2 \  = V_{cc} \frac{R_2}{(R_1+R_2)} - I_B \frac{R_1 R_2}{(R_1+R_2)} \approx V_{cc} \frac{R_2}{(R_1+R_2)} provided I_B << I_2 = V_B / R_2 \. Also V_B = V_{be} + I_ER_E \ For the given circuit, I_B =\frac { \frac {V_{CC}}{1+R_1/R_2} - V_{be} } {( \beta + 1)R_E + R_1 \parallel R_2 }. Merits: Unlike above circuits, only one dc supply is necessary. Operating point is almost independent of β variation. Operating point stabilized against shift in temperature. Demerits: In this circuit, to keep IC independent of β the following condition must be met: I_C = \beta I_B = \beta \frac { \frac {V_{CC}}{1+R_1/R_2} - V_{be} } {( \beta + 1)R_E + R_1 \parallel R_2 } \approx \frac { \frac {V_{CC}}{1+R_1/R_2}- V_{be}} {R_E} , which is approximately the case if ( \beta + 1 ) R_E >> R_1 \parallel R_2 where R1 || R2 denotes the equivalent resistance of R1 and R2 connected in parallel. As β-value is fixed for a given transistor, this relation can be satisfied either by keeping RE fairly large, or making R1||R2 very low. If RE is of large value, high VCC is necessary. This increases cost as well as precautions necessary while handling. If R1 || R2 is low, either R1 is low, or R2 is low, or both are low. A low R1 raises VB closer to VC, reducing the available swing in collector voltage, and limiting how large RC can be made without driving the transistor out of active mode. A low R2 lowers Vbe, reducing the allowed collector current. Lowering both resistor values draws more current from the power supply and lowers the input resistance of the amplifier as seen from the base. AC as well as DC feedback is caused by RE, which reduces the AC voltage gain of the amplifier. A method to avoid AC feedback while retaining DC feedback is discussed below. Usage: The circuit's stability and merits as above make it widely used for linear circuits. Voltage divider with AC bypass capacitor[edit source | editbeta]

Voltage divider with capacitor The standard voltage divider circuit discussed above faces a drawback - AC feedback caused by resistor RE reduces the gain. This can be avoided by placing a capacitor (CE) in parallel with RE, as shown in circuit diagram. Emitter bias[edit source | editbeta]

Emitter bias When a split supply (dual power supply) is available, this biasing circuit is the most effective, and provides zero bias voltage at the emitter or collector for load. The negative supply VEE is used to forward-bias the emitter junction through RE. The positive supply VCC is used to reverse-bias the collector junction. Only two resistors are necessary for the common collector stage and four resistors for the common emitter or common base stage. We know that, VB - VE = Vbe If RB is small enough, base voltage will be approximately zero. Therefore emitter current is, IE = (VEE - Vbe)/RE The operating point is independent of β if RE >> RB/β Merit: Good stability of operating point similar to voltage divider bias. Demerit: This type can only be used when a split (dual) power supply is available.