--- In BITX20@..., Rob Frohne <frohro@w...> wrote:
Hi All,
I have been doing some Spice simulations, and after playing around
with it a bit, it is evident that using microwave transistors will
improve things a lot over the HF general purpose transistors. The
culprit is C sub mu, the capacitance between the collector and the
base due to the reverse biased collector base junction. The problem
is made worse by having both the transmit and receive transistors
capacitance in parallel. This provides more parallel current
feedback than we want at higher frequencies. Simulating using
microwave transistors shows that the 50 ohm input impedance goes up
to almost 100 MHz, but with the general purpose HF transistors, you
only get to a very maximum of 10 MHz (often less), and then the
C sub mu messes things up.
That is indeed true but in practice transistors like the 2n3904
and 2n/pn2222A easily perfom to 20+ mhz. These devices have a
FT or 300-350mhz. A rule of thumb is to stay below FT/10 for
predictable feedback amplifier operation at medium gains. For
example 2n3904 has Ft of 300mhz (min) and nominal HFE of 100
at low frequencies or DC. At 30mhz the effective AC hfe is
only 10 so gains of greater than 10 (20db voltage) are not
likely with broadband circuits. This is due not only to C-B
feedback but also C-E capacitance. The bias currents, working
impedences and feedback amounts do interact. At lower impedences
(50 input and 50 to 200ohm output) the devices perform well.
Even with small degradation the cost of these devices are often
un the less than 10 cents(US) range make them attractive. I usually
source 2n3904, 2n3906, PN2222 types in quantities of 20 for 6
cents(US) each.
So for this design based on experience devices in the 2n3904 and
2222 are good for the IF, LO and maybe the RF up to 20m (14mhz).
A device like the 2n5179 would be advised for the RF amp(Q1)as
is both lower noise and has an FT in excess of 1ghz. Others
that work well are 2N3866, 2n5109, 2sc2570. Devices characterized
for CATV use are excellent.
Things to look for in an IF transistor for this circuit, good HFE at
10-20ma (over 100 is advised). An FT of not less that 300mhz is
adaquate and there are many 2n, and 2SC devices that easily meet
this.
For the RF amp (Q1) the noise figure of the transistor is important.
most good VHF and uhf devices are low noise. However the 2n3904
is surprizingly good (typical 4-5db) for low (below 20mhz) HF use.
I've used 2n3904, 2n2222A with excellent results as high
as 40mhz for circuits such as IF amp, buffer amplifiers and
general use wide band amplifier to 30mhz. Another device the
2n4124 has been used for 6m as amplifier(wideband) and oscillator.
The trick if there is any is recognizing that trying to achieve more
gain at working frequncy than the device will resonably yeild is
moving toward instability and requires a different topology than a
wideband feeback amplifier, or a better device.
An aside: if you going to analyse the amplifiers also anaylyse their
interaction with the crystal filter. You will find that there are
other first order and second order effects as the filter band edges
are approaced and the balanced mixers are not overly fond of
mistermination. These are as significant as C sub mu interactions.
One of the ugly facts of feedback amplifiers as used is that output
impedence interacts with input impedence (and also reverse) and as a
result gain also changes. Crystal filters as you approach band edges
and out of band undergo significant changes in termination phase and
resistance.
Allison,
KB1GMX {in any project there is: Good, Fast, Cheap, pick any two.}
