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From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: Transistor amplifiers v. frequency in SPICE
Date: Sat, 11 Jan 2003 21:24:08 -0500
NNTP-Posting-Date: Sat, 11 Jan 2003 21:23:38 EST
Organization: Bell Sympatico
Paul Burridge wrote:
[... see original for CKT file]
I have to admire your energy and persistence. SPICE is not easy to learn,
and you are doing a good job of asking the right questions.
A couple of quick comments. Your SPICE should be able to plot the AC
response curve instead of having to do each frequency manually. What
version are you using?
The emitter and collector resistors are way too high for the frequency
response you are asking. MicroCap calculates the ft at 1.72MHz under
these conditions. Here are some suggestions you might want to try:
Change the resistors to the following values:
R1 = 1k
R8 = 1k
R7 = 750
R2 = 470
Change C2 to 1uF.
Now plot the frequency response. It should improve, but it still falls
You should be able to find the operating point information in the
printout, and it should indicate the transistor parameters such as
collector, base and emitter current, and also the beta and ft. These are
rough guides to how the transistor is functioning.
Try changing to a higher frequency transistor like the MMBR941:
.MODEL MMBR941 NPN(BF=110 VAF=80 VAR=8.0 RC=.54 RB=.08 RE=.009 IKF=35M
+ ISE=0.45E-14 TF=0.111E-10 TR=0.80E-09 ITF=0.20E-01 VTF=0.33E+01
+ CJC=0.074E-12 CJE=0.064E-12 XTI=3.0 NE=1.5 ISC=0.15E-14 EG=1.11
+ XTB=1.5 BR=2.86 VJC=0.75 VJE=0.75 IS=0.50E-15 MJC=0.33 MJE=0.33 XTF=4.0
+ IKR=0.35E-01 KF=0.1E-14 NC=1.7 FC=0.50 RBM=.06 IRB=0.50E-02 XCJC=0.5)
Now you should see a flat frequency response past 1GHz. Don't believe it
for a minute:)
Add a 50 ohm resistor in series with the signal source. The frequency
response falls off dramatically.
Add a 5pF cap from the collector to ground to simulate stray trace and
pin capacity. The frequency response falls even faster.
Add a 1pf from the collector to the base. The frequency response falls
These are simple ways to see how a circuit is working, and to get a
feeling for the real-world effect of strays. It tells you for wideband
circuits, you need low impedance which means high current.
CMOS designers can get away with much lower operating currents since they
work with much lower internal capacitances, and rarely have to drive
external pins with high frequency analog signals.
But first thing is to get SPICE to plot the AC response curves for you.
Lets see if we can help!
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