BJT Biasing PDF

# BJT Biasing ## Fixed Bias We need to establish a particular VCE and IC for the DC analysis, capacitor C1 and C2 can be replaced by open circuit. IB = (VCC-VBE)/RB; IC=BIB+ICEO Here a regulated DC power supply VCC minus fixed at in voltage of the transistor VBE provide the necessary forward bias...

# BJT Biasing ## Fixed Bias We need to establish a particular VCE and IC for the DC analysis, capacitor C1 and C2 can be replaced by open circuit. IB = (VCC-VBE)/RB; IC=BIB+ICEO Here a regulated DC power supply VCC minus fixed at in voltage of the transistor VBE provide the necessary forward bias of the E-B junction. For a fixed value of RB, the transistor is driven by a fixed DC base current (IB). Hence known as **Fixed Bias**. If Q1 is replaced by Q2, the Q-point will not be the same. Beta of the different transistors vary each other. IC = BIB+(1+B)ICBO The ICBO doubles for every 10°C change in temperature. So as T↑, IC↑ hence Q-point changes. Suppose due to a small change in temperature, IC increases. Hence ICE increases. Heat dissipation increases. Hence T↑. IC↑→T↑→IC↑.... ultimately transistor will burn. This process is know as **thermal runaway**. IC = BIB+(1+B)ICBO IC = B(VCC-VBE)/RB and IB = (VCC-VBE)/RB In the above equation, IC is independent of RC If the transistor is in active region. VCC = ICRC + VCE and VCE=VCC-ICRC. Hence as RC↑, ICE↑, That is transistor is approaching towards the saturation region. Q-point moves to the left side with constant IB and IC. Hence to keep the BJT in active region we should not increase RC beyond a certain limit. Once transistor goes into saturation, VCE = VCEsat under this condition. IC = (VCC-VCESAT)/RC ## Self Bias ckt [voltage divider] If IC↑, IE=IC+IB↑. Then drop across RE↑ le. VE↑. Hence VBE↓. Since the output circuit comprises of a voltage divider circuit with resistors R1 and R2 in series, the biasing scheme is named as Voltage divide bias. Here the voltage drop across R2 provides the necessary bias to the transistor input section. Now we can replace the left portion of the line AB by Thevenin equivalent circuit. The Thevenin voltage VTH is the voltage drop across the resistor R2. To calculate RTH, the voltage source VCC (connected with R1) is shorted and results to the equivalent circuit of the figure. RTH = R1||R2 and VTH = R2VCC/R1+R2. This voltage provides the necessary bias to the base of the ckt named as VBB. VBB = VTH = R2VCC/R1+R2 ## Large Signal Model of BJT In active region, BEJ is forward biased and it is similar to a PN junction diode. The current drawn from collector and flowing into the emitter is depends on VBE. Hence we can modelled by a voltage controlled current source. More accurately, IC=BIB+ICEO; ICEO = (HB)IB Signal is arbitrarily large and the model is valid in forward active region. Let's build an amplified. Vo = -KR Vin We have IC= Is exp(VBE/VT) Let us replace this black box with a BJT Typical value of JS= 5x10^-16 A IC = -VO/RL VO = -RLIC = -RL[JS (exp(VBE/VT))] VBE = 10mV; VT = 26mV, JS =5x10^-16A. Vo = -RL X 2.35X10^-16A. Let us need Vo=100mv so what should be the value of RL? RL = 4.25X10^2 Let's add an additional battery to the mike having value 750mV. Let mike output is zero. Then VBE-750mV. Now Let's apply an input from microphone le 10mV in our example. So VBE changes to 760mV. Let we need Vo = 100mV. Find out RL = 100mV/780uA. while microphone in action Biasing: Providing Proper voltage and Current in the absence of the signal so that transistor can amplify. The bias value chosen for VBE, IC....ete Observations * A BJT can act as a voltage dependent current source. ie, Collector current changes in response to its base emitter voltage * The operating point determines how transistor responds to the input. Which operating point is better?? Case-1 IC= 780uA- 0.0785 530uA gmi = 10mV/173uA Case-2 IC= 802/173uA = 0.45 10mV change. gmi = 10mV/3.6uA Case 2 have stronger response for microphone input. As VBE bias voltage ↑, collector Current become more sensitive to VBE voltage. # Transconductance The derivative of current with respect to the VBE is called transconductance gmi = dIC/dVBE unit of gm is Siemens. gm = d/dVBE (Is exp(VBE/VT)) = (Is/VT) exp(VBE/VT) gm = (IC/VT) exp(VBE/VT) With no bias, IC=0, gm=0 le, No amplification. For amplification, we need certain gm, certain IC, certain VBE. Trade off between gm and power consumption. A ‘small’ change in VBE means a small fraction of VT. # Measurement of gm In forward active region, Vary VBE with a small amount and measure IC keeping VCE constant. # Small Signal Model of BJT In Small Signal operation of transistor, the external signal is small compared to VT. le, Signal is perturbs the bias (operating) point by only a small amount. VBE = Vo+Vm sinwt IC = Is exp(VBE/VT) If IC changes by a small value compared to the bias value of the collector current, then it is called small signal operation. IC0= Is exp(Vo/VT) exp(Vosinwt/VT) ; Assume Vsinwt <<1, le, Vm <<VT then by using the identity, e^x = 1+x if x<<<1. Hence IC ≈ Is exp(Vo/VT) (1+Vosinwt/VT) = IC0 + ICoVosinwt/VT, we have gmi = (ΔIC/ΔV) = gmi IC ≈ IC0+ gmiVm sinwt; if IC perturbation is Smaller. If voltage fluctuations is very small, then IC can be treated as a straight line. le, Output current fluctuation = constant X input fluctuations. IC= IC0+gmVin, we have IB= IC/B+ gmiVin/ B. IB= IC0/B + gmiVin/B can be replaced by a resistance Large signal model (only bias signal). In general, we replace Vin=Vm and B/gmo = 1/gm In small signal model, we need to set all constant voltage sources to zero. le, only perturbation need to be considered. How to incorporate early effect in ssm? The early effect can be mathematically expressed as: IC = [Is exp(VBE/VT)] / (1+VCE/VA) . VA = Early voltage. # Method of small signal model derivation 1. Assume the transistor is biased at a certain operating point. 2. Apply a voltage change between two terminals and measure the resulting current changes. 3. Model the current change by a proper electric device. gm = dIC/dVBE = d/dVBE (Is exp(VBE/VT) (1+VCE/VA)) = (Is / VT) exp(VBE/VT)(1+VCE/VA) = IC/VT (1+VCE/VA) Consider IC varies with VCE, let VCE → VCE+ΔV so that IC → IC+ΔIC IC+ΔIC = (Is exp(VBE/VT) (1+VCE+ΔV/VA) ΔIC = (Is exp(VBE/VT) ΔV/VA) / (1+VCE/VA) Hence IC is proportional to ΔVC. If VBE is constant then ΔICαΔVC. We can replace this relation with a resistor. ro = ΔVCE/ΔIC = VA exp(VBE/VT) / IC = ro (output resistance) Hence complete small signal model considering early effect is: # Field Effect transistor(FET) FETs are 2nd generation transistors which are widely used in integrated circuits (ICs). * FETs are majority carrier device * FETs are unipolar device ie, either electron con holes constitute the carrent flow (Not both) * FETs are voltage controlled devices * Current flow is due to the drift and not because of diffusion. * Compared to the BJT, fabrication is easy. # JFET (Junction Field Effect Transistor) The general structure of JFET is shown below # MOSFET (Metal Oxide Semiconductor Field Effect transistor) Voltage V1 is applied between conductive plate and P-type semiconductor. Positive charge on conductive plate attracts some free electrons of P-type to the surface. These electrons are called channel. ie, by applying V1, we have formed a channel in the device. The charge stored in the capacitor is given by Q=CV. If V1↑ →Q↑[more +ve charges and negative charges]. Ie, electron density of the channel increases. If t↓, capacitance C↑, and Q↑. Here also electron density of the channel increases. By changing V1 or t, we can change the electron density in the channel. Now let us apply another voltage V2 as shown. If V2 is applied, current flows from A to B [Conventional direction of the current]. Now if we increase V1, the electron density of the channel ↑ so current increases between A and B. Ie, resistance of the channel reduces. Now we have something similar to a voltage dependent current source. # MOSFET Structure: In old generation, metal (Aluminum) was used for gate. Now a day Poly silicon is used. Polysilicon is not an excellent conductor but moderately good conductor. Here heavily doped polysilicon with low resistivity is used. nt and p+ To have ohmic contact we need heavily doped semiconductor. MOSFET has 4 terminals 1) Drain 2) Source 3) Gate 4) Substrate/Body/Bulk. Mos structure is symmetrical with respect to the source and Drain. Today the insulation thickness (tox) is in the order of 15 to 18 A°. Non Crystaline silicon (Polysilicon) with heavy doping (for low resistivity) exhibits better fabrication and physical properties. Note: with n type source/Drain and P-type substrate, this transistor operates with electrons rather than holes and is therefore called n-type Mos (NMOS) device.