Application Notes

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Application Note
AN10174-01
A Low Impedance PIN Diode Driver Circuit with Temperature Compensation
Two Philips BAP50-05 PIN diodes are used in
an RF attenuator with a low impedance driver
circuit to significantly decrease the rise and
fall times. A standard attenuator with an
unspecified driver is shown in Figure 1. Each
of the two PIN diodes operates as an RF
resistor whose value is controlled by the DC
current1. The signals reflect off of the diodes
and through the 3 dB hybrid in a way to add in
phase. The amount of signal that is reflected
off the diodes depends on the resistance value.
In this circuit, the diodes are operated from
several hundred ohms down to a value
approaching 50 ohms, where there is no
reflection and thus maximum attenuation.
speed. Insertion loss is generally not important
in this application, and the dynamic range
required may be only 8 to 10 dB. When this is
true, it is possible to achieve a large
improvement in speed.
In driving a PIN diode attenuator, conflicting
requirements arise from speed, linearity, and
temperature compensation.
For the best
speed, a low impedance source (<50 ohms) is
required; for linearity and temperature
compensation, a current source is by far the
best, especially if it is desired to go to
maximum resistance (lowest current) in the
PIN diodes. Figures 2 and 3 show current,
voltage, and attenuation for the circuit of
Figure 1 in two different formats (linear and
log x axis), with a current source for the
driver.
0
0.8
-5 S21
C1 is required for RF bypass, and typically
might be 10-100 pF when working in the GHz
range. An application for this attenuator
circuit is a fast gain controllers in predistorted
and/or feedforward amplifiers, where the
circuit is required to change attenuation in tens
of nS, where C1, C2, and C3 can limit the
0.7
V1
-10
0.6
-15
0.5
-20
0.4
0
0.05 0.1 0.15 0.2 0.25 0.3 0.35
Input Current , mA
1
Although the BAP50-05 contains two diodes, only one
per package is used for mechanical layout reasons.
Figure 2.
1
V1, Volts
S21, dB
Figure 1. Commonly Used Attenuator. Diodes
are BAP50-05.
transistors), the ratio of the two currents is
fixed for all currents (over many decades), and
is controlled by the voltage offsets applied to
them (with respect to each other). This
principle is used in translinear analog
multipliers, of which the Gilbert cell
multiplier is a type.
2
At medium attenuation, the PIN diode
resistance is in the region of several hundred
ohms, and current is in the region of 10-100
uA. The control impedance3 (impedance of
KT
. If driven by a
the diodes) is Z =
qI
current source, such as a current output DAC,
the source impedance is high and the total
impedance is determined by the diodes. The
risetime will be limited by the inevitable
capacitances (illustrated by C5).
0
S21
0.7
V1
-10
0.6
-15
0.5
-20
0.01
0.4
0.1
Input Current , mA
Figure 3.
V1, Volts
S21, dB
-5
0.8
1
In this circuit, the offset is adjusted with V2,
which is only some tens of millivolts.
Operating the device like this is similar to
circuits where the base and collector are tied
together to form a diode. The collector to
emitter voltage is less than the base to emitter
voltage, in magnitude. VCE is roughly 0.65 V.
This is acceptable, without resorting to a
negative supply for the collector, because
there is still several hundred mV of margin
from the standpoint of device saturation.
Q1 is thermally tied to the PIN diodes by
virtue of their proximity, providing a first
order temperature compensation. Q1 thus is
operating as a log circuit converting current to
voltage in a way that linearizes the
attenuation.
If the diodes are driven from a voltage source
(not shown), the speed is very fast, but the
attenuation is highly nonlinear and is highly
temperature dependent.
Shunting the PIN Diodes
Figure 4 shows a circuit which maintains a
low impedance in the PIN circuit, to keep the
rise and fall times short, but linearizes the
circuit to some extent and is temperature
compensated. Only one diode is shown for
simplicity.
Figure 4. Transistor Shunt. V2 is < 200 mV.
The complete circuit is shown in Figure 5. The
hybrid is a surface mount Anaren Xinger
1D1304-3. Figures 6 and 7 show the current,
voltage, and attenuation characteristics. Note
that the input current is much higher than with
the original circuit (Figures 2 and 3). This
reduces efficiency but it is desirable from a
standpoint of keeping the total impedance low.
Operation is as follows: Q1 operates as a
diode and absorbs most of the current from the
current source. It is shown below that for two
diodes in parallel (whether formed by PINs or
2
Actually two diodes in parallel, but for analysis we
will consider one.
3
Not to be confused with RF impedance.
2
Refer to Figure 4. From basic diode equations,
the currents in the PIN diode and Q1 are:
qV 1
I1 = I S1 (e KT − 1)
Figure 5 . Circuit with Two Diodes and Hybrid.
D5 and D6 are Philips BAS50-04. Q1 is
PMBT3906.
I 2 = βI S 2 (e
S21
-10
0.6
-15
0.5
-20
V1, Volts
S21, dB
0.7
V1
For voltages over a few millivolts, the
exponential terms in (1) and (2) dominate the
“1”, and the equations can be simplified to
0.4
0
5 10 15 20 25 30 35
Input Current , mA
I1 = I S 1e
Figure 6. Circuit of Figure 5 (measured).
0
(2)
q is the electron charge, 1.602E-19,
K = Bolzmann’s constant, 1.381E-23
T = temperature in degrees K
IS1 = Saturation current for the PIN diode
IS2 = Saturation current for the base junction
of the transistor
V1 - V2 = the base to emitter voltage of the
transistor (V2 < 0)
q
≈ 40 at room temperature
KT
0.8
-5
− 1)
where
Capacitors C6 and C8 are essentially in
parallel with C5 from a standpoint of the drive
circuitry.
0
q (V 1−V 2 )
KT
(1)
0.8
qV1
KT
I 2 = βI S 2 e
(3)
q (V1 −V 2 )
KT
(4)
S21
0.7
V1
-10
0.6
-15
0.5
-20
0.4
100
0.1
1
10
Input Current , mA
Then, the ratio of the currents is:
V1, Volts
S21, dB
-5
qV1
I1
=
I2
I S 1e KT
βI S 2e
q (V1 −V2 )
KT
=
I S1
−qV2
βI S 2e KT
=
I S1
βI S 2e −40V2
(5)
To the extent that β is constant with
temperature4, we see that the current ratio is
Figure 7. Same as Figure 6 with Log Scale.
β is certainly not constant with temperature, but this
is a second order effect, not nearly as strong as the
direct temperature relationship as with the base emitter
4
Relationship of the Diode
and Transistor Currents
3
dependent only on V2, which, stated another
way, the current in the PIN diode is a fixed
percentage of the total input current. There is
first order temperature compensation, by
virtue of the parallel tracking of the two diode
junctions.
Adjustment
V2 controls the amount of current that Q1
draws relative to the total current It. At low
voltages (50 mV), Q1 does not draw much
current relative to It, and the speed benefit will
be minimal. However, the dynamic range is
the highest, as shown in Figure 8. If lower
dynamic range is acceptable, V2 can be
upwards of 150 mV, where the impedance is
lower and the speed benefit will be the largest.
Of course, using different types of devices for
Q1 and the diodes may require different
values of V2.
Further, we can set the current ratio to an
arbitrary amount by setting the base voltage
V2.
If β = 50, and IS1 = IS2 (by way of
example only), and we want to set the PIN
diode current to 1% of the total current, we
have
.01 =
1
50e − 40V2
so V2 = - .0173.
(6)
25
Dynamic Range, dB
By having a relatively large current in Q1, the
dynamic impedance that the current source
dV1
sees, defined by
becomes much lower,
dI T
dominated by the lower impedance of the Q1.
For a general pn junction this impedance is
KT
Z=
. Thus, in the circuit of Figure 1,
qI
with no shunt transistor, the PIN diodes
operate at perhaps 10 to 100 uA (total for two
diodes), and the impedance ranges from 2500
to 250 ohms.
20
15
10
5
0
0
50
100
150
200
|V2|, mVDC
Figure 8. RF Dynamic Range.
In the circuit of Figures 4 and 5, the PIN
diodes operate at the same 10 to 100 uA, but
the impedance for the parallel combination of
Q1 and the two diodes is 25 to 2.5 ohms5.
Conclusion
A current controlled RF attenuator driver
circuit has been shown which has the speed
advantage of a low impedance (<50 ohm)
driver, and the linearity advantage of a high
impedance (current) driver. This is done by
shunting the PIN diodes with a base-emitter
junction of a transistor, which carries the bulk
(e.g. 99%) of the driver current, lowering the
impedance. The current divides itself between
the transistor and the PIN diodes in a constant
proportion. The current sharing percentage is
settable with the base voltage. Temperature
compensation on a first order basis is inherent
from the tracking of the devices. The tradeoff
is a lower efficiency, the circuit now requiring
Risetimes
In Figure 1, if all the capacitances C1, C2, and
C3 add up to 100 pF, the worst case risetime,
which occurs at the lowest current, will be RC
= 2500*100E-12 = 250 nS. In contrast, the
circuit of Figure 5, the worst case risetime is
25*100E-12 = 2.5 nS.
voltage (Angelo, “Electronics: BJTs, FETs and
Microcircuits”, McGraw Hill 1969.)
5
Neglecting the series emitter resistance of the
transistor which might be 1-2 ohms.
4
10 to 20 mA of drive, as opposed to 100 uA
for the simpler circuit. The current is in the
range of many DACs (current output types)
and this circuit lends itself well to that
application. For application in an envelope
restoration loop such as is found in
predistorted amplifiers, the dynamic range of
8 to 10 dB is acceptable.
5
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