Zetex Variable Capacitance Diodes
Neil Chadderton
Introduction
The advent of varactor diodes has made a h u g e i m p a c t i n m a n y a r e a s o f electronic design, which is only too evident in todays consumer products. Formerly, where bulky or unreliable mechanical methods were used, the size, reliability and excellent tracking abilities of the varactor has led to smaller, cheaper and more elaborate circuitry, previously impossible to attain.
Zetex manufacture an extensive range of Variable Capacitance diodes that are processed using ion-implantation techniques to assure accurate doping levels, and hence produce the exacting junction profiles necessary for high performance devices. An overall capacitance range of approximately 1pF to 200pF assures a broad applications base, enabling designs operating from kHz and extending into the microwave region. Product variability within specification is comparable to, or better t h a n , c o m p e t i t o r s d e v i c e s ; t h e Hyperabrupt series, for example, are available to 5% tolerance on nominal capacitance, due to close targeting during fabrication. Furthermore, Zetex are capable of matching (either to device type or customer specifications) or
banding on capacitance parameters, as required. They are available in surface m o u n t p a c k a g e s t h a t e x h i b i t l o w inductance ensuring a wide frequency application, and assure environmental endurance and mechanical reliability. This application note gives some basic background information, examines the important parameters and specifications for the Zetex range of devices, and suggests a few application circuit examples.
Background
The varactor diode is a device that is processed so to capitalise on the properties of the depletion layer of a P-N diode. Under reverse bias, the carriers in each region (holes in the P type and electrons in the N type) move away from the junction, leaving an area that is depleted of carriers. Thus a region that is essentially an insulator has been created, and can be compared to the classic parallel plate capacitor model. The effective width of this depletion region increases with reverse bias, and so the capacitance decreases. Thus the depletion layer effectively creates a v o l t a g e d e p e n d e n t j u n c t i o n capacitance, that can be varied between the forward conduction region and the reverse breakdown voltage.
Different junction profiles can be p r o d u c e d t h a t e x h i b i t d i f f e r e n t Capacitance-Voltage characteristics. The Abrupt junction type for example, shows a small range of capacitance due t o i t s d i f f u s i o n p r o f i l e , a n d a s a consequence of this is capable of high Q a n d l o w d i s t o r t i o n , w h i l e t h e Hyperabrupt variety allows a larger change in capacitance for the same r a n g e o f r e v e r s e b i a s . S o c a l l e d Hyper-hyperabrupt, or octave tuning variable capacitance diodes show a large change in capacitance for a r e l a t i v e l y s m a l l c h a n g e i n b i a s voltage.This is particularly useful in battery powered systems where the available bias voltage is limited.The varactor can be modelled as a variable capacitance (Cjv), in series with a resistance (Rs). (Please refer to Figure 1).
The capacitance, Cjv, is dependent upon the reverse bias voltage, the junction area, and the doping densities of the semiconductor material, and can be described by:
T h e s e r i e s r e s i s t a n c e e x i s t s a s a c o n s e q u e n c e o f t h e r e m a i n i n g undepleted semiconductor resistance, a contribution due to the die substrate, a n d a s m a l l l e a d a n d p a c k a g e c o m p o n e n t , a n d i s f o r e m o s t i n determining the performance of the device under RF conditions.
This follows, as the quality factor, Q, is given by:
S o , t o m a x i m i s e Q , Rs m u s t b e minimised. This is achieved by the use of an epitaxial structure so minimising the amount of high resistivity material in series with the junction.
RS
Cjv
Figure 1
Common Model for the Varactor Diode.
(
)
Cjv Cj 0 1 = +Vr N ϕ Where: Cj0 = Junction capacitance at 0VCjv = Junction capacitance at applied
bias voltage Vr
Vr = Applied bias voltage
= Contact Potential
N = Power law of the junction or slope
factor. Q 2 fC R = π 1 jv s Where:
Cjv = Junction capacitance at applied bias
voltage Vr
Rs = Series Resistance
NOTE: Zetex has produced a set of SPICE models to enable designers to simulate their circuits in SPICE, PSPICE and similar simulation packages. The models use a version of the above capacitance equation and so the model parameters may also be of interest for other software packages. Information is also provided to allow inclusion of parasitic elements to the model. These models are available on request, from any Zetex sales office.
Important Parameters
This section reviews the important characteristics of varactor diodes with particular reference to the Zetex range of variable capacitance diodes.
The characteristic of prime concern to the designer is the Capacitance-Voltage relationship, illustrated by a C-V curve, and expressed at a particular voltage by Cx, where x is the bias voltage. The C-V curve summarises the range of useful capacitance, and also shows the shape of the relationship, which may be relevant when a specific response is required. Figures 2a, 2b and 2c show
families of C-V curves for the 830 hyperabrupt series, 930 hyper-hyperabrupt series and the 950 low voltage hyperabrupt series. Obviously, the choice of device type depends upon the application, but aspects to consider include: the range of frequencies the circuit must operate with, and hence an appropriate capacitance range; the available bias voltage; and the required response.
Figure 2b
Typical Capacitance-Voltage Characteristics for the 930 Series 931 930 932 933 934 1 10
Reverse Voltage, Vr (Volts)
1 10 100 100 1 1 0.1 10 100 10 200 836 835 834 833 832 830 829 Figure 2a
Typical Capacitance-Voltage Characteristics for the 830 Series
Capacitance v Reverse Voltage
VRReverse Voltage (V) C J Capacitance (pF) 100 10 1 0.1 1 10 ZV953V2 ZMDC953 ZV952V2 ZV952V2 Tj= 25°C f = 1MHz Figure 2c
Typical Capacitance-Voltage Characteristics for the 950 Series
T h e c a p a c i t a n c e r a t i o , c o m m o n l y expressed as Cx/Cy (where x and y are bias voltages), is a useful parameter that shows how quickly the capacitance changes with applied bias voltage. So, for an Abrupt junction device, a C2/C20 figure of 2.8 may be typical, whereas a C2/C20 ratio of 6 may be expected for a Hyperabrupt device. This feature of the Hyperabrupt variety can be particularly important when assessing devices for battery-powered applications, where the bias voltage range may be limited. In this instance, the 930 series that feature a better than 2:1 tuning range for a 0 to 6V bias may be of particular interest. The quality factor, Q, at a particular condition is a useful parameter in assessing the performance of a device with respect to tuned circuits, and the resulting loaded Q.
Zetex guarantee a minimum Q at test conditions of 50MHz, and a relatively low VRof 3 or 4V, and ranges 100 to 450 depending on device type (see Product Range Tables).
The specified VR is very important in assessing this parameter, because as well as the C-V dependence as detailed previously, a significant part of the series resistance (Rs), is due to the remaining undepleted epitaxial layer, which is also dependant upon VR. This Rs-VRrelationship is shown in Figure 3 f o r t h e Z C 8 3 0 , Z C 8 3 3 a n d Z C 8 3 6 Hyperabrupt devices, measured at frequencies of 470MHz, 300MHz and 150MHz respectively, and also serves to illustrate the excellent performance of Zetex Variable Capacitance Diodes at VHF and UHF. 600 1 10 100 Reverse Voltage VR(V) 100 200 400 300 500 700 TEMP = 25°C TYPICAL ZC830 @ 470MHz @ 300MHz ZC833 @ 150MHz ZC836 R s (m ) ⍀ Figure 3
Typical RSv VRRelationship for ZC830 Series
Also of interest, with respect to stability, i s t h e t e m p e r a t u r e c o e f f i c i e n t o f capacitance, as capacitance changes with VR, and is shown for the ranges in figures 4a, and 4b.
The reverse breakdown voltage, V(BR) also has a bearing on device selection, as this parameter limits the maximum VR that may be used when biasing for minimum capacitance. Zetex variable capacitance diodes typically possess a V(BR) of 25V for the 830 series and 12V for the 930, 950 series.
The maximum frequency of operation will depend on the required capacitance and the series resistance (and hence useful Q), that is possessed by a particular device type, but also of consequence are the parasitic components exhibited by the device package. These depend on the size, material, and construction of the package. For example, the Zetex SOT-23 package has a typical stray capacitance of 0.08pF, and a total lead inductance of 2.8nH. These low values allow a wide
frequency application, for example, the ZC830 and ZC930 series, configured as series pairs are ideal for low cost microwave designs extending to 2.5GHz and above.
Applications
Variable capacitance diodes can be used in any tuned circuit application where previously mechanical methods were utilised, and provide a size, cost and performance advantage. This section briefly examines some typical examples of varactor application.
The conventional simple tuned circuit of Figure 5a can also be effected by the varactor version as in Figure 5b, where capacitor C1 isolates the DC bias. The choice of varactor for such a circuit depends on the tuning range and hence capacitance, with particular attention b e i n g p a i d t o t h e C - V r e g i o n approaching 0V, as this may introduce non-linearity and poor Q. Another similar approach is to use the series configuration shown in Figure 5c which, while allowing a lower apparent diode 600 1 10 30 100 200 400 300 500 700 Tj = 0-100°C TYPICAL 0 p p m/ °C Figure 4a
Temperature Coeffcient of Capacitance v VRfor
the ZC830 Series. 10 1 0.1 Vr-Reverse Voltage (V) 600 800 1000 1200 400 200 0 1400 Tj=25 to 125° C TYPICAL pp m / °C Figure 4b
Temperature Coefficient of Capacitance v VRfor
c a p a c i t a n c e , a l s o p r e v e n t s R F rectification at low values of diode bias therefore inhibiting generation of intermodulation products, and also simplifies bias requirements.
A practical front-end for an FM receiver is shown in Figure 6, with each stage b e i n g t u n e d b y i t ’ s o w n d i o d e . Multi-stage units are therefore possible
without the severe tracking errors, and the massive size penalty inherent to mechanical mechanisms.
As the tuning is now controlled by a voltage, the inevitable inclusion of the microprocessor and memory in many modern receivers has allowed band-s c a n n i n g a n d band-s t a t i o n band-s t o r a g e b y p r o d u c i n g t h e c o n t r o l v o l t a g e 470 22p 3n9 4K7 BFS17H 4n7 15K 47p 47K 47p D2 10p 47K 3K3 22n 47p 2N3819 22 3n9 47p D3 47K 10p BFS17H 10K 2p2 22p 2K7 27K 3n9 10n D1 10p +12V SUPPLY VOLTAGE 0-30V I.F. OUTPUT 10.7 MHz D1-D3 ZC823 ZMV823 CONTROL VOLTAGE AERIAL Figure 6
Typical FM receiver Front End employing Variable Capacitance Diodes.
(c) (b) (a) C1 DC Bias DC Bias Figure 5
automatically. It is noteworthy that the control voltage of any system must possess good voltage and temperature stability.
Perhaps the largest effect on consumer products due to the varactor has been the development of varactor based VHF and UHF tuners for televisions. These have enabled solid state non-mechanical designs that are smaller, more reliable and allow elaborate features such as remote control and station searching. Figure 7 shows a typical circuit of a UHF tuner incorporating varactor diodes. Stage matching is effected by the bias trimmers RV1-5, and allows adjustment remote from the actual tuning element; the mechanical equivalent being to add padding capacitance, or to bend the vanes on an air-spaced capacitor.
Such a tuner can, using the large capacitance range of Hyperabrupt varactor diodes, tune the whole channel r a n g e o f b a n d s I V a n d V (470MHz-850MHz).
Another common application for the varactor is as the frequency controlling e l e m e n t i n a V o l t a g e C o n t r o l l e d Oscillator (VCO). There are many applications for such circuits, either as stand alone units or as part of a phase locked loop in a frequency synthesiser. This latter method is commonly utilised in radio telecommunications and for the tuning stages in satellite receivers. Closely allied to these are functions such a s f r e q u e n c y p u l l i n g o n c r y s t a l o s c i l l a t o r s , n a r r o w b a n d F M a n d t e m p e r a t u r e c o m p e n s a t i o n o f frequency within an oscillator, all of which can benefit from a varactor diode based design. D2 D4 RV3 RV2 D5 RV5 RV1 D1 CONTROL VOLTAGE RV4 SUPPLY VOLTAGE D3 SUPPLY VOLTAGE AERIAL INPUT AGC I.F. OUTPUT Figure 7
Typical UHF Variable Capacitance Diode Tuner Module
D1 - D5 - ZC831 or ZMV831 or ZV831V
In its basic form, and for a square wave output, a common method of providing a VCO is to use logic gates coupled as shown in Figure 8; this particular design giving a 1 to 1.25MHz range. For a transistor design, a VCO can be realised
by modifying the Clapp configuration as shown in Figure 9. For this example, the frequency can be varied over a 75 to 150MHz range for a 0-30V control voltage. ZC834 0.15u 22K 39p 4K7 4K7 O/P 74C00 VC
& & &
Figure 8
Basic Form of VCO using Logic Gates.
ZMV834 22K 820p 820 47p 3K3 BFS17H 390 1n L1 47p
on 1/4" Former
1
+12V
O/P
NOTE : L = 5T
VC
Figure 9Figure 10 shows a similar configuration for a 1GHz VCO. Obviously at this frequency, circuit construction is critical and capable of producing large tuning range changes. For this example, the transistor was mounted in a slot in a small ground plane configured board, and the other components supported by short leads. This produced a signal level of -5dBm with a 10dB PAD into 50 ohms. The second harmonic was observed at -35dB down from the fundamental. O t h e r t e c h n i q u e s u s i n g l u m p e d c o m p o n e n t s a n d Z e t e x v a r i a b l e capacitance diodes are capable of output at 1.5-2.5GHz, typically for tuning units for satellite receivers.
Figure 10
1GHz VCO Producing -5dBm with a 10dB PAD into 50 . 2 x ZC831 or 1 x ZMDC831 33K 33k 5K6 L1 22p BFR90 150 27 27 33 20K 10u VC NOTE : L = 1 cm long 1mm dia O/P +12V 1n 4p7 2p2 2p2 1
Low Phase Noise Capability
D u e t o t h e g e o m e t r i c f e a t u r e s employed in the Zetex range of variable capacitance diodes, these products exhibit extremely low values of leakage current (typically less than 20 pA which enables excellent low phase noise performance within VCXO circuitry.
T h e v a r a c t o r d i o d e a l s o e n a b l e s frequency multipliers to be produced that exhibit very high conversion efficiency, a zero DC power requirement, and low component count. Figure 11a shows the general appearance of a varactor tripler, and consists of input and output matching, and a trap for the unwanted second harmonic. Figure 11b shows a similar circuit for a 100-300MHz
tripler using a ZC830, and includes a bandpass filtered output and a trap for the fundamental.
OUTPUT INPUT
Figure 11a
Varactor Tripler with Marching Input and Output Stages.
ZC830
Figure 11b
Appendix
Zetex Variable Capacitance Diode Product Range.
The tables presented within this Appendix illustrate the standard abrupt, hyperabrupt and hyper-hyperabrupt ranges available with respect to datasheet characteristics and package style. In addition, Zetex can also supply to competitors’ type numbers and customers’ specific requirements.
SOD523 SOD323 SOT23 SOT323
DUAL SOT23 DUAL Capacitance VR= 2V, f = 1MHz Capacitance Ratio C2/C20 at f = 1MHz Q VR= 3V f = 50MHz
TYP (pF) Min Max Min
ZMV829A ZC829A 8.2 4.3 5.8 250 ZV829BV2(1) ZMV829B ZC829B ZMDC829B(1) 8.2 4.3 5.8 250 ZMV830A ZC830A 10.0 4.5 6.0 300 ZV830BV2(1) ZMV830B ZC830B ZMDC830B(1) 10.0 4.5 6.0 300 ZMV831A ZC831A 15.0 4.5 6.0 300 ZV831BV2 ZMV831B ZC831B ZMDC831B 15.0 4.5 6.0 300 ZMV832A ZC832A 22.0 5.0 6.5 200 ZV832BV2 ZMV832B ZC832B ZMDC832B 22.0 5.0 6.5 200
ZMV833A ZC833A ZDC833A 33.0 5.0 6.5 200
ZMV833B ZC833B ZMDC833B(1)
33.0 5.0 6.5 200
ZMV834A ZC834A ZDC834A 47.0 5.0 6.5 200
ZMV834B ZC834B 47.0 5.0 6.5 200 ZMV835A ZC835A 68.0 5.0 6.5 100 ZMV835B ZC835B 68.0 5.0 6.5 100 ZC836A 100.0 5.0 6.5 100 ZMV836B ZC836B 100.0 5.0 6.5 100 Note:
Suffix A diodes are 10% tolerance. Suffix B diodes are 5% tolerance. (1)
Planned for future release. Please contact your local Zetex sales office for more details.
HIGH PERFORMANCE HYPER-HYPER ABRUPT
SOD523 SOT323 DUAL Capacitance
VR= 0.5V, f = 1MHz Capacitance VR= 1.5V, f = 1MHz Capacitance C0.5/C2.5 f = 1MHz Q VR= 0.5V f = 50MHz
Min (pF) Min Max Min Min
ZV950V2 ZMDC950(1) 9.5 6.3 7.8 2.0 250 ZV951V2(1) ZMDC951(1) 15.0 9.4 11.6 2.0 250 ZV952V2 ZMDC952(1) 19.00 12.7 15.7 2.0 250 ZV953V2 ZMDC953 45.0 30.0 37.0 2.0 200 (1)
Planned for future release. Please contact your local Zetex sales office for more details.
SOD523 SOT323 SOT23 Capacitance
VR= 1V, f=1MHz Capacitance VR= 2.5V, at f=1MHz Capacitance VR = 4V f=1MHz Q VR = 4V f=50MHz
Min (pF) Min Max Min (pF) Min
ZV930V2(1) ZMV930 ZC930 8.70 4.30 5.50 2.90 200 ZV931V2 ZMV931 ZC931 13.50 6.50 7.80 4.00 300 ZV932V2 ZMV932 ZC932 17.00 8.50 10.50 5.50 200 ZV933V2 ZMV933 ZC933 42.00 18.00 27.00 12.00 150 ZMV933A ZC933A 42.00 20.25 24.75 12.00 150 ZMV934 ZC934 95.00 40.00 65.00 25.00 80 ZMV934A ZC934A 95.00 47.25 57.75 25.00 80
(1) Planned for future release. Please contact your local Zetex sales office for more details.
