द्विध्रुवी ट्रांजिस्टर की अभिनति

द्विध्रुवी ट्रांजिस्टर से बने प्रवर्धक को समुचित ढंग से अभिनत करना पड़ता है तभी यह अपेक्षित ढंग से कार्य कर सकता है अन्यथा विविध प्रकार की समस्याएँ आ सकतीं हैं। बीजेटी को अभिनत करने के लिए कई तरह के नेटवर्क प्रयोग किए जाते हैं, जिनकी अपनी-अपने गुणदोष हैं। अलग-अलग बीजेटी से निर्मित प्रवर्धकों में प्रायः प्रतिरोधों के विभिन्न विन्यासों की सहायता से अभिनत किया जाता है जबकि एकीकृत परिपथों में इसकी अपेक्षा अधिक उन्नत अभिनति का उपयोग किया जाता है (बैण्ड-गैप वोल्टेज रिफरेन्स, करेन्ट मिरर आदि)।

अभिनति की डिजाइन में इस बात को सुनिश्चित किया जाता है कि ट्रांजिस्टर का ताप बदलने पर, ट्रांजिस्टर का β बदलने पर या थोड़ा अलग β वाला ट्रांजिस्टर लगाने पर, या डीसी सप्लाई के वोल्टेज में थोड़ा परिवर्तन होने पर भी ट्रांजिस्टर का संचालन बिन्दु उतना अधिक न बदले ताकि ट्रांजिस्टर अपने सक्रिय क्षेत्र में ही बना रहे और अपेक्षित ढंग से कार्य कर सके।

ट्रांजिस्टर के 'संचालन बिन्दु' का अर्थ है- जब इन्पुट संकेत न लगा हो उस स्थिति में ट्रांजिस्टर का डीसी कलेक्टर-एमिटर वोल्टता (V_ce) और कलेक्टर धारा (I_c)। इसे 'बायस-बिन्दु' , 'क्विसेन्ट बिन्दु' या 'क्यू-बिन्दु' भी कहते हैं।

• (१) क्लास-ए प्रवर्धक को इस प्रकार अभिनत किया जाता है कि ताप, बीटा, बायस सप्लाई आदि के एक सीमा में बदलने पर भी सक्रिय क्षेत्र में बना रहे।

• (२) बायस बिन्दु को स्थिर रखने में ऋणात्मक प्रतिपूर्ति (निगेटिव फीडबैक) बहुत उपयोगी सिद्ध होता है।

• (३) ताप बदलने पर ट्रांजिस्टर की इनपुट अभिलाक्षणिक (VBE के साथ IB का परिवर्तन) बहुत अधिक बदल जाता है। इससे निपटने का समुचित प्रबन्ध करना चाहिए अन्यथा संचालन-बिन्दु बहुत अधिक बदल सकता है। निगेटिव फीडबैक का उपयोग तथा ट्रांजिस्टर को समुचित हीटसिंक पर लगाने से यह उद्देश्य पूरा किया जा सकता है।

ट्रांजिस्टर एम्प्लिफायरों को क्लास-ए में अभिनत करने के लिए मुख्यतः निम्नलिखित अभिनति परिपथों का उपयोग किया जाता है-

1. स्थिर अभिनति (Fixed bias)
2. कलेक्टर-से-बेस अभिनति (Collector-to-base bias)
3. स्थिर अभिनति के साथ एमिटर में प्रतिरोध (Fixed bias with emitter resistor)
4. वोल्टता-विभाजक अभिनति (Voltage divider bias)
5. एमिटर अभिनति (Emitter bias)

इस अभिनति को 'बेस बायस' अथवा 'फिक्स्ड रेजिस्टैन्स बायस' भी कहते हैं। सामने के चित्र में कलेक्टर और बेस दोनों के लिए एक ही डीसी सप्लाई (जैसे बैटरी) का उपयोग किया गया है, किन्तु दो अलग-अलग सप्लाई भी प्रयोग किए जा सकते हैं।

इस चित्र के सन्दर्भ में-

V_cc = I_bR_b + V_be

अतः,

I_b = (V_cc − V_be)/R_b

For a given transistor, V_be does not vary significantly during use. As V_cc is of fixed value, on selection of R_b, the base current I_b is fixed. Therefore, this type is called fixed bias type of circuit.

Also for given circuit,

V_cc = I_cR_c + V_ce

Therefore,

V_ce = V_cc − I_cR_c

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.

अतः

I_c = βI_b

we can obtain I_c as well. In this manner, operating point given as (V_ce,I_c) can be set for given transistor.

लाभ:

• It is simple to shift the operating point anywhere in the active region by merely changing the base resistor (R_b).
• A very small number of components are required.

अलाभ:

• The collector current does not remain constant with variation in temperature or power supply voltage. Therefore, the operating point is unstable.
• Changes in V_be will change I_B and thus cause I_E 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.^[]

उपयोग:

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.

This configuration employs negative feedback to prevent thermal runaway and stabilize the operating point. In this form of biasing, the base resistor ${\displaystyle R_{\text{b}}}$ is connected to the collector instead of connecting it to the DC source ${\displaystyle V_{\text{cc}}}$. So any thermal runaway will induce a voltage drop across the ${\displaystyle R_{\text{C}}}$ resistor that will throttle the transistor's base current.

From Kirchhoff's voltage law, the voltage ${\displaystyle V_{{\text{R}}_{\text{b}}}}$ across the base resistor ${\displaystyle R_{\text{b}}}$ is

${\displaystyle V_{{\text{R}}_{\text{b}}}=V_{\text{cc}}\,-\,{\mathord {\overbrace {(I_{\text{c}}+I_{\text{b}})R_{\text{c}}} ^{{\text{Voltage drop across }}R_{\text{c}}}}}\,-\,{\mathord {\overbrace {V_{\text{be}}} ^{\text{Voltage at base}}}}.}$

By the Ebers–Moll model, ${\displaystyle I_{\text{c}}=\beta I_{\text{b}}}$, and so

${\displaystyle V_{{\text{R}}_{\text{b}}}=V_{\text{cc}}-(\overbrace {\beta I_{\text{b}}} ^{I_{\text{c}}}+I_{\text{b}})R_{\text{c}}-V_{\text{be}}=V_{\text{cc}}-I_{\text{b}}(\beta +1)R_{\text{c}}-V_{\text{be}}.}$

From Ohm's law, the base current ${\displaystyle I_{\text{b}}=V_{{\text{R}}_{\text{b}}}/R_{\text{b}}}$, and so

${\displaystyle \overbrace {I_{\text{b}}R_{\text{b}}} ^{V_{{\text{R}}_{\text{b}}}}=V_{\text{cc}}-I_{\text{b}}(\beta +1)R_{\text{c}}-V_{\text{be}}.}$

Hence, the base current ${\displaystyle I_{\text{b}}}$ is

${\displaystyle I_{\text{b}}={\frac {V_{\text{cc}}-V_{\text{be}}}{R_{\text{b}}+(\beta +1)R_{\text{c}}}}}$

If ${\displaystyle V_{\text{be}}}$ is held constant and temperature increases, then the collector current ${\displaystyle I_{\text{c}}}$ increases. However, a larger ${\displaystyle I_{\text{c}}}$ causes the voltage drop across resistor ${\displaystyle R_{\text{c}}}$ to increase, which in turn reduces the voltage ${\displaystyle V_{{\text{R}}_{\text{b}}}}$ across the base resistor ${\displaystyle R_{\text{b}}}$. A lower base-resistor voltage drop reduces the base current ${\displaystyle I_{\text{b}}}$, which results in less collector current ${\displaystyle I_{\text{c}}}$. Because an increase in collector current with temperature is opposed, the operating point is kept stable.

लाभ:

• Circuit stabilizes the operating point against variations in temperature and β (i.e. replacement of transistor).
• Circuit stabilizes the operating point (as a fraction of ${\displaystyle V_{\text{cc}}}$) against variations in ${\displaystyle V_{\text{cc}}}$.

अलाभ:

• Although small changes in β are OK, large changes in β will greatly change the operating point। ${\displaystyle R_{\text{b}}}$ must be chosen once β is known fairly accurately (perhaps within ~ 25%), yet the variability of β between "identical" parts is often larger than this.
• In this circuit, to keep ${\displaystyle I_{\text{c}}}$ independent of ${\displaystyle \beta }$, the following condition must be met:

${\displaystyle I_{\text{c}}=\beta I_{\text{b}}={\frac {\beta (V_{\text{cc}}-V_{\text{be}})}{R_{\text{b}}+R_{\text{c}}+\beta R_{\text{c}}}}\approx {\frac {(V_{\text{cc}}-V_{\text{be}})}{R_{\text{c}}}}}$

which is the case when

${\displaystyle \beta R_{\text{c}}\gg R_{\text{b}}.}$

• As ${\displaystyle \beta }$-value is fixed (and generally unknown) for a given transistor, this relation can be satisfied either by keeping ${\displaystyle R_{\text{c}}}$ fairly large or making ${\displaystyle R_{\text{b}}}$ very low.
  • If ${\displaystyle R_{\text{c}}}$ is large, a high ${\displaystyle V_{\text{cc}}}$ is necessary, which increases cost as well as precautions necessary while handling.
  • If ${\displaystyle R_{\text{b}}}$ 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 ${\displaystyle R_{\text{b}}}$ causes an AC feedback, reducing the voltage gain of the amplifier. This undesirable effect is a trade-off for greater Q-point stability. However, a T (R-C-R) network can be used to reduce the AC feedback, which however poses a heavier load on the collector than the simple feedback resistor. At higher frequencies a R-L feedback network can be used, however, it will introduce peaking into the frequency response at various points.

उपयोग: The negative feedback also increases 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.

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

${\displaystyle V_{R_{b}}=V_{cc}-I_{e}R_{e}-V_{be}}$

From Ohm's law, the base current is

${\displaystyle I_{b}={\frac {V_{R_{b}}}{R_{b}}}}$

The way feedback controls the bias point is as follows. If V_be is held constant and temperature increases, emitter current increases. However, a larger I_e increases the emitter voltage V_e = I_eR_e, which in turn reduces the voltage V_Rb across the base resistor. A lower base-resistor voltage drop reduces the base current, which results in less collector current because I_c = β I_b. Collector current and emitter current are related by I_c = α I_e with α ≈ 1, so the increase in emitter current with temperature is opposed, and the operating point is kept stable.

Similarly, if the transistor is replaced by another, there may be a change in I_c (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,

${\displaystyle I_{b}={\frac {V_{cc}-V_{be}}{R_{b}+(\beta +1)R_{e}}}}$

Advantages:

The circuit has the tendency to stabilize operating point against changes in temperature and β-value.

Disadvantages:

• In this circuit, to keep I_C independent of β the following condition must be met:

${\displaystyle I_{C}=\beta I_{b}={\frac {\beta (V_{cc}-V_{be})}{R_{b}+(\beta +1)R_{e}}}\approx {\frac {(V_{cc}-V_{be})}{R_{e}}}}$

which is approximately the case if

${\displaystyle (\beta +1)R_{e}\gg R_{b}}$

• As β-value is fixed for a given transistor, this relation can be satisfied either by keeping R_e very large, or making R_B very low.
  • If R_e is of large value, high V_cc 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, R_e causes AC feedback which reduces the voltage gain of the amplifier.

उपयोग:

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

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. The voltage divider configuration achieves the correct voltages by the use of resistors in certain patterns. By manipulating the resistors in certain ways you can achieve more stable current levels without having β value affect it too much.

In this circuit the base voltage is given by:

${\displaystyle V_{B}=\ }$ voltage across ${\displaystyle R_{2}\ }$ ${\displaystyle =V_{cc}{\frac {R_{2}}{(R_{1}+R_{2})}}-I_{b}{\frac {R_{1}R_{2}}{(R_{1}+R_{2})}}}$

${\displaystyle \approx V_{cc}{\frac {R_{2}}{(R_{1}+R_{2})}}}$ provided ${\displaystyle I_{b}<<I_{2}=V_{b}/R_{2}\ }$.

Also ${\displaystyle V_{B}=V_{be}+I_{e}R_{e}\ }$

For the given circuit,

${\displaystyle I_{b}={\frac {{\frac {V_{cc}}{1+R_{1}/R_{2}}}-V_{be}}{(\beta +1)R_{e}+R_{1}\parallel R_{2}}}.}$

लाभ:

• Unlike above circuits, only one dc supply is necessary.
• Operating point is almost independent of β variation.
• Operating point stabilized against shift in temperature.

अलाभ:

• In this circuit, to keep I_C independent of β the following condition must be met:

${\displaystyle I_{c}=\beta I_{b}=\beta {\frac {{\frac {V_{cc}}{1+R_{1}/R_{2}}}-V_{be}}{(\beta +1)R_{e}+R_{1}\parallel R_{2}}}\approx {\frac {{\frac {V_{cc}}{1+R_{1}/R_{2}}}-V_{be}}{R_{e}}},}$

which is approximately the case if

${\displaystyle (\beta +1)R_{e}>>R_{1}\parallel R_{2}}$

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 R_E fairly large, or making R₁||R₂ very low.
  • If R_E is of large value, high V_cc 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 V_c, 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 V_be, 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 R_e, which reduces the AC voltage gain of the amplifier. A method to avoid AC feedback while retaining DC feedback is discussed below.

उपयोग:

The circuit's stability and merits as above make it widely used for linear circuits.

The standard voltage divider circuit discussed above faces a drawback – AC feedback caused by resistor R_E reduces the gain. This can be avoided by placing a capacitor (C_e) in parallel with R_e, as shown in circuit diagram. The result is that the DC operating point is well controlled, while the AC-gain is much higher (approaching β), rather than the much lower (but predictable) value of ${\displaystyle R_{\text{c}}/R_{\text{e}}}$ without the capacitor.

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 V_ee is used to forward-bias the emitter junction through R_e. The positive supply V_cc 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,

V_b − V_e = V_be

If R_b is small enough, base voltage will be approximately zero. Therefore, emitter current is,

Ie = (V_ee − V_be)/R_e

The operating point is independent of β if Re >> R_b/β

Advantages:

Disadvantages:

This type can only be used when a split (dual) power supply is available.

Class B and AB amplifiers employ 2 active devices to cover the complete 360 deg of input signal flow. Each transistor is therefore biased to perform over approximately 180 deg of the input signal. Class B bias is when the collector current I_c with no signal is just conducting (about 1% of maximum possible value). Class-AB bias is when the collector current I_c is about 1/4 of maximum possible value. The class-AB push–pull output amplifier circuit below could be the basis for a moderate-power audio amplifier.

प्रवर्धक का एक व्यावहारिक परिपथ

Q3 is a common emitter stage that provides amplification of the signal and the DC bias current through D1 and D2 to generate a bias voltage for the output devices. The output pair are arranged in class-AB push–pull, also called a complementary pair. The diodes D1 and D2 provide a small amount of constant voltage bias for the output pair, just biasing them into the conducting state so that crossover distortion is minimized. That is, the diodes push the output stage into class-AB mode (assuming that the base-emitter drop of the output transistors is reduced by heat dissipation).

This design automatically stabilizes its operating point, since overall feedback internally operates from DC up through the audio range and beyond. The use of fixed diode bias requires the diodes to be both electrically and thermally matched to the output transistors. If the output transistors conduct too much, they can easily overheat and destroy themselves, as the full current from the power supply is not limited at this stage.

A common solution to help stabilize the output device operating point is to include some emitter resistors, typically an ohm or so. Calculating the values of the circuit's resistors and capacitors is done based on the components employed and the intended use of the amplifier.

$${\displaystyle I_{B}={\frac {V_{BB}-0.7V}{R_{B}}}}$$

इसमें मान रखने पर,

$${\displaystyle I_{B}={\frac {5V-0.7V}{500k\Omega }}=8,6\mu A}$$

अब,

$${\displaystyle I_{C}=\beta *I_{B}=100*8,6\,\mu A=0,86\,mA}$$

अब आउटपुट परिपथ में (बैटरी, R,sub>c, ट्रांजिस्टर का Vsub>ce) लगाने पर,

$${\displaystyle {\begin{aligned}V_{CE}&=V_{CC}-R_{C}*I_{C}\\&=12-0,86\,mA*2K\Omega \\&=10,28V\end{aligned}}}$$

• अभिनति
• लघु संकेत मॉडल (Small signal model)
• द्विध्रुवी जंक्शन ट्रांजिस्टर
• मॉसफेट

• Bias – from Sci-Tech Encyclopedia
• Electrical Engineering Training Series: Types of bias