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Unformatted text preview: 1 Bipolar Junction Transistors (BJTs) (13) 2 BIPOLAR JUNCTION TRANSISTORS (BJTs) 5.6 SMALLSIGNAL OPERATION & MODELS (Transistor as an Amplifier) To operate as an amplifier, the transistor must be biased in the active region: EBJ will be forward biased and CBJ will be reverse biased and if you recall i C v CE graph , BJT will operate to the right side of the saturation region i.e. V CE > 0.2 volts, or so. 3 BIPOLAR JUNCTION TRANSISTORS (BJTs) 5.6 SMALLSIGNAL OPERATION & MODELS (Transistor as an Amplifier) Biasing a BJT implies, establishing a constant DC current in the emitter (or the collector) by utilizing DC voltage sources etc ( even once dc source can do the job). By designing, it is desired to keep this dc current least sensitive to : (a) Variations in Temperature (b) Value of , etc. Let us look at the operation / functioning of the following circuit: In the Figure 5.48 (a), on the right we observe that V BE forward biases the EBJ. V CC through R C reverse biases the CBJ. V be is the input signal voltage represented as a source. This signal is to be amplified (in the form of current or voltage amplification) 4 BIPOLAR JUNCTION TRANSISTORS (BJTs) 5.6 SMALLSIGNAL OPERATION & MODELS (Transistor as an Amplifier) DC Conditions: If we consider first the dc bias conditions by setting v be = 0 ( Short circuiting the voltage source v be ) we have following circuit: And we can write as per our knowledge about BJT in active mode, that V C should be sufficiently greater than V B to allow for reasonable and symmetrical signal (i c ) swing about the Qpoint (details later) 5 BIPOLAR JUNCTION TRANSISTORS (BJTs) 5.6.1 The Collector Current and the Transconductance If a signal v be is applied as shown in Fig. 5.48(a), the total instantaneous baseemitter voltage v BE becomes: v BE = V BE + v be Correspondingly, the collector current becomes: Use of equation 5.78 yields, i C = I C e (v be / V T ) (5.82) , now if v be << V T This equation may be approximated to be: i C I C (1 + v be / V T ) (5.83) Here we have expanded the exponential in Eq. (5.82) in a series and retained only the first two terms. This approximation, which is valid only for v be less than approximately 10 mV, is referred to as the smallsignal approximation. Under this approximation the total collector current is given by Eq. (5.83) and can be rewritten 6 BIPOLAR JUNCTION TRANSISTORS (BJTs) 5.6.1 The Collector Current and the Transconductance Under this approximation the total collector current is given by Eq. (5.83) and can be rewritten as: i C = I C + (I C / V T ) v be (5.84) Thus the collector current is composed of the dc bias value I C and a signal component i c , i c = (I C / V T ) v be (5.85) This equation relates the signal current in the collector to the corresponding baseemitter signal voltage. It can be rewritten as: i c = g m v be ( 5.86 ), where g m is called the transconductance, and from Eq 5.85 it is given by and from Eq 5....
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This note was uploaded on 01/29/2011 for the course EE 203 taught by Professor Gp.(r)muzaffarali during the Fall '10 term at College of E&ME, NUST.
 Fall '10
 Gp.(R)MuzaffarAli
 Amplifier, Transistor

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