# vb.net barcode reader source code Figure 101 Superposition of AC and DC signals in Software Drawing QR in Software Figure 101 Superposition of AC and DC signals

Figure 101 Superposition of AC and DC signals
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Figure 101 depicts the appearance of the collector current iC (t) when ICQ = 5 10 3 A and IC (t) = 05 10 3 sin t A Imagine the collector curves of Figure 919 magni ed about the Q point Figure 102 graphically depicts the interpretation of each of the h parameters relative to the operating point of the BJT The parameter hie is approximately equal to the ratio VBE / IB in the neighborhood of the Q point; Figure 102(a) illustrates how this parameter is equal to the reciprocal of the slope of the IB -VBE curve at the operating point Physically, this parameter represents the forward resistance of the BE junction The parameter hre is representative of the fact that the IB -VBE curve is slightly dependent on the actual value of the collector-emitter voltage, VCE However, this effect is virtually negligible in any applications of interest to us Thus, we shall assume that hre 0 Figure 102(b) depicts the shift in the IB -VBE curves represented by hre A typical value of hre for VCE 1 V is around 10 2 The parameter hf e is approximated in Figure 102(c) by the current ratio IC / IB This parameter represents the current gain of the transistor and is approximately equivalent to the parameter introduced earlier For the purpose of our discussion, and hf e will be interchangeable, although they are not exactly identical
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IB ( A)
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VCE = VCEQ
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IB ( A) VCE Q IBQ
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VCEQ
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VBEQ 0 VBE (a) Interpretation of h ie VBE (V) 0
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VBEQ VBE (V) VBE (b) Interpretation of h re IC (mA)
IC (mA)
IC Q ICQ IB IBQ
IC ICQ
IBQ Q
0 0 VCEQ VCE (V) (c) Interpretation of h fe
VCEQ VCE
VCE (V)
(d) Interpretation of hoe
Figure 102 Graphical interpretation of h parameters
The parameter hoe may be calculated as hoe = IC / VCE from the collector characteristic curves, as shown in Figure 102(d) This parameter is a physical indication of the fact that the IC -VCE curves in the linear active region are not exactly at; hoe represents the upward slope of these curves and therefore has units of conductance (S) Typical values of hoe are around 10 5 S We shall often assume that this effect is negligible To be more precise, the h parameters are de ned by the following set of equations: hie = vBE iB iC vCE iC iB vBE vCE ( )
C + IB B + VBE E VCE
(101)
hoe =
VCEQ
(102)
hf e =
A A V V
(103)
hfe IB hie
C 1 hoe
hre =
(104)
+ hre VCE E
VCEQ
The circuit of Figure 103 illustrates the small-signal model side by side with the BJT circuit symbol Representative parameters for a small-signal transistor are listed in Table 101
Figure 103 h-parameter small-signal model for BJT
10
Transistor Ampli ers and Switches
Table 101 h parameters for the 2N2222A BJT Parameter hie (k ) hre ( 10 4 ) hf e hoe ( S) 50 5 Minimum 2 Maximum 4 8 300 35
iC RC C iB RB B + vCE E RE VCC
+ ~ VB
iB = IBQ + IB iC = ICQ + IC vCE = VCEQ + VCE
Figure 104 BJT ampli er
To illustrate the application of the h-parameter small-signal model of Figure 103, consider the transistor ampli er circuit shown in Figure 104 This circuit is most readily analyzed if DC and AC equivalent circuits are treated separately To obtain the DC circuit, the AC source is replaced by a short circuit The resulting DC circuit is shown in Figure 105 The DC circuit may be employed to carry out a Q-point analysis similar to that of Examples 94 and 96 Since our objective at present is to illustrate the AC circuit model for the transistor ampli er, we shall assume that the DC analysis (ie, selection of the appropriate Q point) has already been carried out, and that a suitable operating point has been established Replacing the DC voltage sources with short circuits, we obtain the AC equivalent circuit of Figure 106 The transistor may now be replaced by its hparameter small-signal model, also shown in Figure 106 We may simplify the model by observing that h 1 is a very large resistance and that if the load resistance oe RL (in parallel with h 1 ) is small (ie, if RL hoe 01), the resistor h 1 in the oe oe model may be ignored The linear AC equivalent circuit makes it possible to take advantage of the circuit analysis techniques developed in s 2 and 3 to analyze the operation of the ampli er For example, application of KVL around the base circuit loop yields the following equation: VB = IB R B + IB hie + (hf e + 1) IB RE (105)