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 (Vce) और कलेक्टर धारा (Ic)। इसे 'बायस-बिन्दु' , 'क्विसेन्ट बिन्दु' या 'क्यू-बिन्दु' भी कहते हैं।

अभिनति से सम्बन्धित कुछ ध्यातव्य बातें
(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।

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

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

स्थिर अभिनति या बेस बायस 

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-

Vcc = IbRb + Vbe

अतः,

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.

अतः

Ic = βIb

we can obtain Ic as well. In this manner, operating point given as (Vce,Ic) 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 (Rb).
 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 Vbe will change IB and thus cause IE 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.

कलेक्टर फीडबैक बायस 

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

From Kirchhoff's voltage law, the voltage  across the base resistor  is

By the Ebers–Moll model, , and so

From Ohm's law, the base current , and so

Hence, the base current  is

If  is held constant and temperature increases, then the collector current  increases. However, a larger  causes the voltage drop across resistor  to increase, which in turn reduces the voltage  across the base resistor . A lower base-resistor voltage drop reduces the base current , which results in less collector current . 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 ) against variations in .

अलाभ:
 Although small changes in β are OK, large changes in β will greatly change the operating point।  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  independent of , the following condition must be met:

which is the case when

As -value is fixed (and generally unknown) for a given transistor, this relation can be satisfied either by keeping  fairly large or making  very low.
 If  is large, a high  is necessary, which increases cost as well as precautions necessary while handling.
 If  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  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.

एमिटर में प्रतिरोध के साथ फिक्स्ड बायस 

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

From Ohm's law, the base current is

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 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 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,

Advantages:

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

Disadvantages:

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

which is approximately the case if

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.

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

वोल्टता विभाजक अभिनति या एमिटर बायस  
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:

 voltage across  
 provided .

Also 

For the given circuit,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 IC independent of β the following condition must be met:

which is approximately the case if

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

 वोल्टता विभाजक, एसी बायपास संधारित्र के साथ 

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. 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  without the capacitor.  

 एमिटर बायस 

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/βAdvantages:Disadvantages:'''

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

क्लास-बी तथा एबी प्रवर्धकों की अभिनति

Signal requirements
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 Ic with no signal is just conducting (about 1% of maximum possible value). Class-AB bias is when the collector current Ic 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.

विविध

किसी NPN बीजेटी के उपयोग का व्यावहारिक  उदाहरण 

 

इसमें मान रखने पर,

अब,

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

इन्हें भी देखें 
 अभिनति
 लघु संकेत मॉडल (Small signal model)
 द्विध्रुवी जंक्शन ट्रांजिस्टर
 मॉसफेट

सन्दर्भ

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

इलेक्ट्रॉनिक इंजीनियरी