# Electronic Devices and Circuits

Topics: Bipolar junction transistor, Input impedance, Output impedance Pages: 2 (441 words) Published: May 4, 2013

UNIT-V SMALL SIGNAL LOW FREQUENCY BJT MODEL 1. List out the advantages of h-parameters. 1. h-parameters are real numbers at audio frequencies. 2. These are easy to measure. 3. h-parameter can also be obtained from the transistor static characteristic curves. 4. h-parameters are convenient to use in circuit analysis and design. 5. A set of h-parameters is specified for many transistors by the manufacturers 2. Draw the h-parameter circuit and its equivalent circuit in CE configuration. h-parameter model

Generalized h-parameter model of an NPN BJT. Replace x with e, b or c for CE, CB and CC topologies respectively. As shown in above diagram, the term "x" in the model represents a different BJT lead depending on the topology used. For common-emitter mode the various symbols take on the specific values as: x = 'e' because it is a common-emitter topology Terminal 1 = Base Terminal 2 = Collector Terminal 3 = Emitter ii = Base current (ib) io = Collector current (ic) Vin = Base-to-emitter voltage (VBE) Vo = Collector-to-emitter voltage (VCE)

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and the h-parameters are given by: hie – The input impedance of the transistor (corresponding to the emitter resistance re). hre – Represents the dependence of the transistor's IB–VBE curve on the value of VCE. It is usually very small and is often neglected (assumed to be zero). hfe – The current-gain of the transistor. This parameter is often specified as hFE or the DC current-gain (βDC) in datasheets. hoe – The output impedance of transistor. This term is usually specified as an admittance and has to be inverted to convert it to an impedance. For the CE topology, an approximate h-parameter model is commonly used which further simplifies the circuit analysis. For this the hoe and hre parameters are neglected (that is, they are set to infinity and zero, respectively). It should also be noted...