IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: EXPRESS BRIEFS , VOL. 52, NO. 1, JANUARY 2005
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Insight Into PIN Diode Behavior Leads to Improved Control Circuit S. C. Bera and P. S. Bharadhwaj
Abstract—This brief offers a fresh insight into the behavior of PIN diodes over temperature. A simple but accurate biasing and control circuit for attenuators and other PIN diode applications is described. The circuit can easily be optimized for a high attenuation setting accuracy over a wide temperature range without using any temperature sensor or compensation circuit.
Caverly and Hiller [3]–[5] have shown that for a given bias cur)th power of the abrent, the RF resistance varies as the ( solute temperature:
Index Terms—Analog multiplexer, attenuator, bias network, control circuit, equiresistance curve, PIN diode, RF resistance.
The RF resistance at different bias currents and temperatures is therefore given by
(2)
(3)
I. INTRODUCTION
A
LTHOUGH PIN diodes have been used for several decades, the design of bias networks for electronically controlled PIN diode based attenuators, phase shifters, etc., has remained something of a black art. This is particularly true when accurate control is required over a wide temperature range. The reason is that the effect of temperature on the RF resistance of PIN diodes was not well understood in the past. In the early 1980’s, Alpha Industrie’s Application Note no. 80 200 [2] had this to say: “We do not label the [Temperature versus RF resistance] curves because we do not yet have enough data and do not want to mislead you. We want you to realize that you will have to do your own experimenting with your diode in your application.” Indeed, it was not until 1993 that Caverly and Hiller [2]–[4] showed that the RF resistance of these diodes th power of the temperature, where the varies as the power m lies between 0 and 2 for practical PIN diodes. In this brief we investigate some mathematical consequences of the above fact and show that a very simple bias network, designed on this principle, can provide 20 dB setting accuracy dB (With constant current biasing the variation would of dB.) over a temperature range of C to C be for practical values of m. This temperature stability is achieved without using a temperature sensor or compensating circuit. We also present a practical implementation and test results for such a circuit. II. PIN DIODE MODEL It has long been known that the RF resistance of a PIN diode, above its cut-off frequency, varies inversely as a power of the forward dc current [1]
Furthermore, the forward dc current is related to the forward voltage by this well-known equation (neglecting a term corresponding to the reverse saturation current) (4) is the reverse saturation current and given by is the electron charge, is Boltzman’s is the Band gap potential of the semiconductor in constant, volts and is the ideality factor of the diode. Equipped with (3) and (4), we can investigate whether a simple bias network can provide accurate attenuation control with a low temperature coefficient. III. ACHIEVING A CONSTANT RF RESISTANCE OVER TEMPERATURE Caverly and Hiller have suggested that a carefully fabricated can be used in an diode having a temperature index attenuator to achieve a zero temperature coefficient when biased at a constant forward current. It seems unlikely, however, that this requirement can be met without sacrificing other aspects of the PIN diode’s performance. In any case we find that typical commercially available diodes have and close to unity, which means that constant-current biasing will not provide acceptable temperature stability. We now discuss what requirements the bias network must meet in order to maintain a constant RF resistance over temperature without imposing narrow constraints on the characteristics is the desired RF resistance and of the diodes used. If the actual resistance at a given current and temperature, then the bias network must ensure that
(1) (5) Manuscript received April 8, 2004. This paper was recommended by Associate Editor A. Apsel. The authors are with the Space Applications Centre, Indian Space Research Organization, Ahmedabad 380 015, India (e-mail:
[email protected]). Digital Object Identifier 10.1109/TCSII.2004.839537
Combining this (5) with (3) and (4), we can write
1057-7130/$20.00 © 2005 IEEE
(6)
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Fig. 1.
IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: EXPRESS BRIEFS , VOL. 52, NO. 1, JANUARY 2005
Fig. 4. Equi-resistance Curves and Load lines for different attenuation settings.
Equiresistance curve and load line.
TABLE I DIODE PARAMETER VALUES
Fig. 2. Equiresistance curves for different
p values.
step. As shown in Fig. 1, the circuit’s open-circuit output voltage (in the sense of Thevenin’s theorem) must be equal to the voltage intercept of the load line, and its Thevenin output resistance must be the reciprocal of the slope. When this exercise is repeated for a number of RF resistance (attenuation) values, we observe the interesting fact that the voltage intercept of the equiresistance curves is almost constant for different attenuation values, and the voltage intercepts of the corresponding optimal load lines are also within a few millivolts of each other. This is shown in Fig. 4 for attenuation values from 2 to 16 dB. This is no mere coincidence, as we can see if we differentiate the constant resistance locus and write the equation for the bias point tangent to the curve at the room-temperature
(7) Fig. 3.
Equiresistance curves for different
m values.
which is the equation of the ideal bias-point locus that will satisfy (5) across the temperature range. We can call this locus the of the RF “equiresistance curve” for this required value resistance. If the constant-resistance V-I locus happens to be highly linear within the temperature range of interest, then the load line of a simple bias circuit can be adjusted to coincide with this curve, and will then maintain the desired RF resistance over temperature as shown in Fig. 1. This is indeed the case for the practical ranges of values of and . Figs. 2 and 3 show these curves for various values of and for an attenuation of 8 dB. Numerical calculations using a typical range of values have confirmed that a temperature stability of less than 0.2 dB C and C. is easily achievable between Once the optimum load line has been determined, it is a simple matter to design the bias network for a given attenuation
Taking this tangent to be our optimum load line, its voltage intercept is (8) The intercept is seen to be practically independent of the selected bias point , implying that the optimum no-load output voltage of the bias circuit is the same for all attenuthis “magic” voltage is very ation values. When of the semiconductor, since close to the bandgap potential the last term is of the order of a few millivolts. In the general , the voltage intercept is given by (8). case when Two types of diode were studied by measuring voltage versus current versus attenuation at various temperatures. The diode parameters in (3) and (4) were extracted by fitting them to the measured data. We have found that the optimum Thevenin in both cases. Table I shows the voltage is slightly less than results of this investigation.
BERA AND BHARADHWAJ: INSIGHT INTO PIN DIODE BEHAVIOR
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TEST RESULTS
TABLE II(a) 2-STAGE ATTENUATOR FOR V (WITH MECHANICAL SWITCH)
OF
= 1:16 V
Fig. 5. Digital electronic control circuit.
TABLE II(b) = 0:989 V TEST RESULTS OF 2-STAGE ATTENUATOR, FOR V (WITH ANALOG MULTIPLEXER, CD4051)
Fig. 6.
Schematic circuit of the single-stage PIN diode attenuator.
Many commercial integrated voltage references are based on the bandgap principle and provide outputs close to the of silicon. These parts, with a provision for minor trimming of the voltage, may be used conveniently as bias sources in this application. IV. PRACTICAL CONTROL CIRCUIT Since the circuit must provide a constant no-load output voltage and a varying output resistance, we can use either 1) a constant voltage source in series with a variable resistor, or 2) a constant voltage source in series with a bank of pre-adjusted resistors, from which one resistor at a time is switched into the circuit. The latter option, which is well suited to digital electronic control, is shown in Fig. 5. The constant voltage is provided by a bandgap reference with associated trimming resistors. The bias control resistor is selected by an analog multiplexer such as the 74HC4051. This part has an ON resistance of a few tens of ohms, which must be taken into account when adjusting the resistor network. Fig. 6 shows the schematic circuit of a single-stage attenuator at C-Band (3900 MHz) that was used to prove the design
principles presented here. A Ku-band attenuator with a similar topology has also been tested. The different series resistors consist of potentiometers adjusted exactly to the theoretically derived optimal values. In practical applications, fixed resistors with values close to the optimum could be used. Very approximately, a 1% change in the resistance will result in a 0.2-dB change in the attenuation at the 20-dB setting, but will not affect the temperature- coefficient of the attenuation. V. TEST RESULTS Table II(a) shows the performance of the attenuator control circuit based on a mechanical rotary switch and Table II(b) with two numbers of type CD4051 analog multiplexer for a two-stage attenuator circuit of diode type HPND4005. The attenuation error is better than 0.2 dB over the temperature range. VI. CONCLUSION A hitherto unexplored mathematical property of the PIN diode has been discussed. This property is exploited in a simple and easily adjusted control circuit that provides excellent setting accuracy and temperature stability. It is shown that this approach can be used for a wide range of practically available diodes. The circuit uses no separate temperature sensor or compensating
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IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: EXPRESS BRIEFS , VOL. 52, NO. 1, JANUARY 2005
mechanism, but responds directly to the junction temperature of the diodes. This prevents any errors caused by temperature gradients, or by self-heating of the diodes due to high RF levels. ACKNOWLEDGMENT The authors would like to acknowledge V. K. Garg, Deputy Director, Satcom-Payload Technology Area, and R. V. Singh, Group Director, Power Amplifier Group, SAC, for their encouragement and support for this work.
REFERENCES [1] “Applications of PIN diodes,” Hewlett-Packard Co., Palo Alto, CA, Applicat. Note No. 922, May 1973. [2] “PIN diode basics,” Alpha Industries, Inc., Woburn, MA, Applicat. Note No. 80200. [3] R. H. Caverly and G. Hiller, “The temperature dependence of PIN diode attenuators,” in Dig. IEEE MTT-S, 1993, pp. 553–556. [4] , “Temperature insensitive PIN diode attenuators,” Appl. Microwave, p. 88, Summer 1993. , “Temperature effects on PIN diode forward bias resistance,” Solid [5] State Electron., vol. 38, no. 11, pp. 1879–1885, 1995.
