ECEE 472/519 RF Electronics
CAD Lab I: Transistor Library Models
To: Dr. Daryoush Li Zhang
From: Stephen Watt
Date Performed: Jan. 24, 2014 Date Submitted: Feb 1, 2014
OBJECTIVE The objective of CAD Experiment Ex periment I was simulation of the static and d ynamic behavior of commercial FET and BJT in various classes of op eration and input-output ports.
THEORY Biasing of FET and BJT amplifiers enable the designer to choose the mode of operation (saturation, linear, cutoff) and the class of operation for the amplifier (A, B, AB, C). Different DC biasing for amplifier circuits can modify the frequency response, linearity, and noise of the circuit. Six different amplifier configurations were examined during the CAD Lab I analysis.
Figure 1. BJT and FET amplifier configurations without bias circuits
EXPERIMENTAL SETUP The student used the curve tracing simulation tools in Agilent ADS to produce the I-V curves for both the Mitsubishi FET and Agilent BJT. The characteristic curves provided the voltage and current information needed to design the DC biasing circuits for the amplifiers. Next, the S parameters for each MGF1302 and HBFP0420 amplifier circuit were calculated.
Figure 2. Agilent ADS tools used to analyze BJT and FET amplifiers
CALCULATIONS & DESIGN Using the provided DC bias conditions, the student calculated the circuit component values to achieve the desired current and voltage levels. AC Bias Circuit Calculations
=
=
(2)
=
1 (2)
=
2.5Ω (2(50))
= 7.96 7.96
1 (2(50)0.5Ω)
= 6.37 6.37
DC Bias Circuit Equations
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[] ]
Case Case Case Case Case
Case Case Case Case Case
CS CG CD
1 2 3 4 5
1 2 3 4 5
= −
=
=
= +1
( − ) =
−
−
= ( ) + = (
+
)
Mitsubishi MGF1302 GD4 FET Agilent HBFP0420 SOT-343 BJT Ids = 40 mA, Vds = 2V Ic = 15 mA, Vce= 8V Ic = 50 mA, Vce= 8V Ids = 20 mA, Vds = 2V Ids = 5 mA, Vds = 1.5V Ic = 0µA, 0µ A, Vce= 8V Ids = 0 mA, Vds = 1.5V Ic = 40 mA, Vce= 8V Ids = 15 mA, Vds = 0.25V Ic = 7 mA, Vce = 0.25 V Table 1. Self-bias cases analyzed for amplifier circuits Mitsubishi MGF1302 GD4 FET Agilent HBFP0420 SOT-343 BJT RS (Ω) RD (Ω) RB (Ω) RC (Ω) RE (Ω) 1.25 73.75 48.8k 185 246.8 15 135 13.9k 58 77.33 110 590 1.0E14 5 6.0E9 3.3E24 9.4E24 18.3k 75 98.6 14.3 325 Table 2. Calculated bias components compone nts for five cases RD (Ω) RS (Ω) R1 (Ω) R2 (Ω) RFC (nH) 1 1 7.96 135 15 7.96 150 4.26k 5.0k 7.96 Table 3. Calculated bias components for Case 2 FET circuits
CC (nF) 6.37 6.37 6.37
CE CB CS
RC (Ω) RE (Ω) R1 (Ω) R2 (Ω) RFC (nH) 352.1 150 7.0k 5.78k 7.96 352.1 150 7.96 172.62 7.0k 7.38k 7.96 Table 4. Calculated bias components for Case C ase 4 BJT circuits
RESULTS FET and BJT Characteristic Curves
CC (nF) 6.37 6.37 6.37
FET and BJT Self Bias Circuits
FET Bias Circuits FET Case 1: Ids = 40 mA, Vds = 2V
FET Case 2: Ids = 20 mA, Vds = 2V
FET Case 3: Ids = 5 mA, Vds = 1.5V
FET Case 4: Ids = 0 mA, Vds = 1.5V
FET Case 5: Ids = 15 mA, Vds = 0.25V
Common Source FET
Common Gate FET
Common Drain FET
BJT Self Bias Circuits BJT Case 1: I c = 15 mA, Vce= 8V
BJT Case 2: I c = 50 mA, Vce= 8V
BJT Case 3: I c = 0µA, 0µ A, Vce= 8V
BJT Case 4: I c = 40 mA, Vce= 8V
BJT Case 5: I c = 7 mA, Vce = 0.25 V
Common Emitter BJT
Common Base BJT
Common Collector BJT
ANALYSIS The BJT amplifier bias conditions and configurations showed the frequenc y response of a high pass filter. The FET amplifier bias conditions and configurations maintained maintained the frequency response of band pass filters. The S11 and S22 of the amplifiers showed response outside of the =1
circle, characteristic of an amplifier. Both the BJT and FET bias conditions demonstrated the linear, saturation, and cutoff operation modes of the devices.
QUESTIONS & HW 1. Active biasing networks are employed to compen sate for the drawbacks of passive biasing networks such as sensitivity to to transistor parameters, and poor temperature stability. Although active biasing offers advantages over passive biasing, additional space and power requirements are introduced.
2. The gain of the class B amplifier a mplifier is higher than the class A amplifier gain, but class B suffers from crossover distortion. The gain of class AB is higher than class A but lower than class B. Based on the ADS simulation results, the three classes of amplifiers appear to function as high pass filters. This would imply that the g ain is larger at higher frequencies than it is at lower frequencies. 3. Three of the main amplifier types that are implemented using different bias points are Class A, Class B, and Class AB. In Class A operation the transistor is biased such that it operates in the linear conduction region during the entirety of the input wave cycle (=360). This means the changes in the amplified output are exactly proportional to the changes in the input. Class A amplifiers are used for oscillators and LNAs. Class B amplifiers amplify only half of the input wave c ycle (=180) and create a large amount of distortion but the efficiency is much improved relative to a Class A amplifier. These amplifiers can be used for RF power amplifiers where increased distortion is not as problematic. Class AB amplifiers are a compromise between Class A and Class B. Th ese amplifiers are less efficient than Class B but more efficient than Class A. 4. The CS amplifier has a relatively high voltage and current gain that is negative, n egative, indicating a 180 phase shift. The CG configuration has the same voltage gain as the CS, except there is no negative sign. The Th e current gain for the CG amplifier is ideally unity. The CD amplifier, or the source follow, has a voltage gain of unity with a high h igh current gain. 5. See attached
CONCLUSION The student used Agilent ADS to analyze the different bias conditions (linear, saturation, cutoff) and six different BJT and FET amplifier configurations (CS, CG, CD, CE, CC, CB). The analysis focused on the Mitsubishi MGF1302 FET and the Agilent HBFP0420 BJT. The different bias conditions enabled operation of class A, class B, and class AB amplifier circuits. The student gained experience with designing DC/AC biasing for RF amplifiers. The student also observed the input/output impedance and the forward power gain for each of the amplifier configurations. For future RF amplifier bias design, more diligence will be taken to ensure the transistors are functioning in the proper mode of o f operation (linear, cutoff, saturation) by using the voltage and current probe features in Agilent ADS.
