10/4/2023 0 Comments Bjt transistor ac analysisThese leads are labeled E, B, and C for emitter, base, and collector, respectively. A lead connects to each of the three regions, as shown. The pn junction joining the base region and the collector region is called the base-collector junction, as indicated in Figure 2(b). The pn junction joining the base region and the emitter region is called the base-emitter junction. The substrate is a physical supporting material for the transistor The term bipolar refers to the use of both holes and electrons as current carriers in the transistor structure. One type consists of two n regions separated by a p region (npn), and the other type consists of two p regions separated by an n region (pnp). Physical representations of the two types of BJTs are shown in Figure 1(b) and 1(c). The three regions are called emitter, base, and collector. The BJT is constructed with three doped semiconductor regions separated by two pn junctions, as shown in the epitaxial planar structure in Figure 1(a). Type of BJT Analysis Bipolar Junction Transistor (BJT) Structure The BJT is used in two broad areas-as a linear amplifier to boost or amplify an electrical signal and as an electronic switch. Two basic types of transistors are the bipolar junction transistor (BJT), and the field-effect transistor (FET), which we will cover later. All of the complex electronic devices and systems today are an outgrowth of early developments in semiconductor transistors. 6) Inverse Resistance Reflection Rule (IRRR): Resistances are reflected from the base to emitter by dividing them by (β+1).The invention of the transistor was the beginning of a technological revolution that is still continuing. Common Collector (CC) or Emitter Follower 5) Resistance Reflection Rule (RRR): The resistor looking into base is (β+1) times the total resistance in the emitter. Basically, there are three types of amplifier: 1. Then, the amplifier can be replaced by its equivalent circuit model. # vo RE R’L vbe gmvbe - + vb ib vo RE vbe gmvbe R’L - + R’s RC 4) An amplifier is characterized by: (a) Gain: voltage gain ( i o v v v A ) current gain ( i o i i i A ) (b) Input resistance (c) Output resistance. 3) Transistor must be biased in middle of active region to allow for maximum signal swing at the output. # Revisit 1) Small signal AC equivalent circuit of BJT beT T be v v v v 1 vbe 5 mV or up to 5mV vi 10 mV 2) The model is the same for npn or pnp transistor since the signal of interest is ac signal. vs 0 vbe 0 The dependent current source vg bem will be open circuit RO = RC (iv) Finding current gain # B S B s E m L s o s B S B s E m L s s E m L o s E s be s m E be m E s s be m be E be s be be s sb be b m be E O m be L R R R R r R r g R voltage gain v v v R R R R r R r g R v R r R g R r v R r R v r v R r g r R r v g R r r R v v g v R r V R v r v v Ri v i gV R v g v R 1( ) 1( ) 1( ) 1( ) ) ] 1 (iii) Finding output resistance RO = the resistance seen by RL First, short circuit the input voltage source. (i) Finding the voltage gain Apply Thevenin: R’s We assume all capacitors are short circuit (small impedances) in ac analysis. # 12V 6k 1k VCC Rs vs vo C 4k 1k RL= 1k C # BCJ is reverse confirming that BJT is working in active 2. DC Analysis: Assume the transistor operates in active mode Apply Thevenin between base and ground: short ckt DC voltage sources and open ckt DC current sources) Perform circuit analysis (EE201) 5. Also, set all independent sources to zero (i. # Replace the transistor by its model 3.Draw the complete ac ckt assuming all capacitors short ckts 4. All capacitors open ckt ind IC and check that the transistor F s working in active mode i Calculate the ac model parameters. the load CE is called bypass capacitor 2. C1 and C2 are called coupling capacitors preventing any dc to go to the source and. Example #1 Find the gain of the BJT amplifier (vo/vi) Observations: 1.
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