|NE5204||Wide-band high-frequency amplifier|
|NE5204 Datasheet PDF : 14 Pages |
Wide-band high-frequency amplifier
THEORY OF OPERATION
The design is based on the use of multiple feedback loops to
provide wide-band gain together with good noise figure and terminal
impedance matches. Referring to the circuit schematic in Figure 17,
the gain is set primarily by the equation:
+ (RF1 ) RE1) ń RE1
which is series-shunt feedback. There is also shunt-series feedback
due to RF2 and RE2 which aids in producing wide-band terminal
impedances without the need for low value input shunting resistors
that would degrade the noise figure. For optimum noise
performance, RE1 and the base resistance of Q1 are kept as low as
possible, while RF2 is maximized.
The noise figure is given by the following equation:
ȡ ƪ ƫȣ rb
ȧȢ ȧȤ NF + 10Log 1 )
where IC1=5.5mA, RE1=12Ω, rb=130Ω, KT/q=26mV at 25°C and
R0=50 for a 50Ω system and 75 for a 75Ω system.
The DC input voltage level VIN can be determined by the equation:
where RE1=12Ω, VBE=0.8V, IC1=5mA and IC3=7mA (currents rated
Under the above conditions, VIN is approximately equal to 1V.
Level shifting is achieved by emitter-follower Q3 and diode Q4,
which provide shunt feedback to the emitter of Q1 via RF1. The use
of an emitter-follower buffer in this feedback loop essentially
eliminates problems of shunt-feedback loading on the output. The
value of RF1=140Ω is chosen to give the desired nominal gain. The
DC output voltage VOUT can be determined by:
where VCC=6V, R2=225Ω, IC2=8mA and IC6=5mA.
From here, it can be seen that the output voltage is approximately
3.1V to give relatively equal positive and negative output swings.
Diode Q5 is included for bias purposes to allow direct coupling of
RF2 to the base of Q1. The dual feedback loops stabilize the DC
operating point of the amplifier.
The output stage is a Darlington pair (Q6 and Q2) which increases
the DC bias voltage on the input stage (Q1) to a more desirable
value, and also increases the feedback loop gain. Resistor R0
optimizes the output VSWR (Voltage Standing Wave Ratio).
Inductors L1 and L2 are bondwire and lead inductances which are
roughly 3nH. These improve the high-frequency impedance
matches at input and output by partially resonating with 0.5pF of pad
and package capacitance.
POWER DISSIPATION CONSIDERATIONS
When using the part at elevated temperature, the engineer should
consider the power dissipation capabilities of each package.
At the nominal supply voltage of 6V, the typical supply current is
25mA (32mA max). For operation at supply voltages other than 6V,
see Figure 3 for ICC versus VCC curves. The supply current is
inversely proportional to temperature and varies no more than 1mA
between 25°C and either temperature extreme. The change is 0.1%
per °C over the range.
The recommended operating temperature ranges are air-mount
specifications. Better heat-sinking benefits can be realized by
mounting the SO and N package bodies against the PC board
1992 Feb 25
Figure 17. Schematic Diagram
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