Preamplifiers
Preamplifier Short Form Catalog in PDF Format
Preamplifiers
This section describes the amplifier circuits recommended for Judson detectors. The PA5, PA6, PA7 and PA9 series preamplifiers are current gain amplifiers recommended for photovoltaic detectors and applications. Voltage mode preamplifiers, for use with our photoconductive detectors, include the PA101, PA8200 and PA300 series preamps. Some general information on our preamplifiers follows:
Noise Sources: Figure 1 shows the various noise sources of the detector/preamp system. Values for the preamp noise sources en, in, Vos and ib are listed in the specification tables for each Judson currentmode preamplifier. The preamp noise sources, together with the detector characteristics, determine the system noise. While a complete analysis of detector system noise is beyond the scope of this guide, the effects of the various noise sources can be summarized by the following approximation:
Total e_{n}(f) = [(e_{n}^{2}/Z_{D}^{2}) + i_{n}^{2} + (4kT/R_{D}) + (4kT/R_{F})]^{1/2} Z_{F}
where k is Boltzmann's constant and T is temperature in degrees Kelvin.
This simplified noise equation provides a good approximation of the total voltage noise density (V/Hz^{1/2}) at the preamplifier output. Note that the noise is dependent on the frequency f, and is normalized to a 1 Hz noise bandwidth. The four terms in the brackets represent the four main sources of current noise:
• Preamplifier noise voltage e_{n} divided by the detector reactance Z_{D}, where
Z_{D} = R_{D}/(1+ (2pf)^{2} C_{D}^{2} R_{D}^{2})^{1/2}
• Preamplifier current noise i_{n}
• Johnson thermal current noise from the detector shunt resistance R_{D}
• Johnson thermal current noise from the preamp feedback resistance R_{F}
The total current noise is then multiplied by the transimpedance gain Z_{F}, where
Z_{F} = R_{F}/(1+ (2pf)^{2} C_{F}^{2} R_{F}^{2})^{1/2}
Analysis of the simplified noise equation shows the following:
• In situations where Z_{D} is large (>10Kohm) the preamplifier current noise i_{n} is more important than the voltage noise e_{n}. This is generally the case when using highimpedance detectors (InSb, cooled Ge, smallarea Ge) at moderate frequencies. Choose a preamp with low i_{n}.
• In situations where Z_{D} is small (<1Kohm), the preamp voltage noise e_{n} becomes more important. This is generally true with lowimpedance detectors (InAs, largearea Ge). Choose a preamp with low e_{n}.
• Larger R_{F} adds less current noise. For highest sensitivity, R_{F} should be greater than R_{D} when practical.
Preamp Noise Figure: A general method for evaluating noise performance of a preamplifier is the noise figure, N_{F}, which indicates what portion of the system noise is caused by the preamp.
N_{F} = 10 log_{10} [Total Noise / Detector Noise]
A perfect preamplifier has a Noise Factor of 0 dB, indicating that the preamp noise contribution is negligible compared to the detector noise. A N_{F} of 0.1 to 3 dB is considered satisfactory. Preamps with N_{F} >3 dB add significant noise to the system. See Fig. 6 for noise figures of Judson transimpedance preamplifiers at 1 KHz.
DC Applications  Offset Drifting: In DC applications, the preamp input bias current I_{b} and input offset voltage V_{os}become important. In an ideal opamp, I_{b} and V_{os} are zero. In reality they have nonzero values. Together with the detector R_{D} they produce a "dark current" I_{D}:
I_{D} = I_{b} + (V_{os}/R_{D})
The DC offset voltage at the preamp output is equal to I_{D} x R_{F}. I_{b} and R_{D} each have a nonlinear dependence on temperature. The offset voltage at the preamplifier output will therefore drift with temperature changes. To minimize offsets and drifting:
For highimpedance detectors, choose a preamp with low Ib. For lowimpedance detectors, choose a preamp with low V_{os}. Consider stabilizing the detector temperature by using one of Judson's integral TEcooler packages. The transimpedance (or currentmode) preamplifier circuit of Fig. 1 is recommended for most PV detector applications, for frequencies up to 1 MHz. It offers lowest noise and best linearity under a wide range of conditions. The characteristics of the opamp circuit maintain the diode near 0V bias. All the photocurrent from the detector essentially flows through the feedback resistor RF. The feedback capacitance CF is added to control gain peaking (Fig. 2). The value of CF depends on the detector capacitance. It is installed at the factory to provide stable preamplifier performance with a particular detector model. The values of RF and CF, together with the detector characteristics RD and CD, determine the overall frequency response of the system (Figs. 3, 4, 7, 8).
PA9 Preamplifier
The PA9 preamplifier is ideal for highfrequency performance with highimpedance photovoltaics such as cryogenically cooled InSb and Ge. The PA9 offers low current noise and ultralow voltage noise. However, its relatively high DC offset voltage makes it less suitable for DC applications than other Judson preamps. The PA9 has fixed gain. When ordered with a detector, the preamp is matched to the detector for maximum gain and sensitivity. Alternatively, the customer may specify gain or minimum required bandwidth. Bandwidth is a function of detector resistance and capacitance as well as preamp gain (Figs. 3 and 4).
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Model 
1st Stage Gain (V/A) 
1st Stage Bandwidth (Maximum) 
PA970  107  DC to 100KHz 
PA960  106  DC to 300KHz 
PA950  105  DC to 750KHz 
PA944  2.5x104  DC to 1MHz 
Typical Specifications Model PA9 Preamplifiers
2nd Stage Gain  20  dB 
Voltage Noise Density @1KHz  6.5  nV Hz1/2 
Voltage Noise from 0.1 to 10 Hz  1.0  µVpp 
Current Noise Density @ 1KHz, 1E7 Gain †  0.04  pA Hz1/2 
Input Offset Voltage  ± 10  mV 
Input Bias Curren  t  ± 1  pA 
Maximum Output  1st stage = 6 2nd stage = 10  Vp 
p 
ance  < 50  W 
Power Requirements  ±12 or ±15 20  VDC mA 
PA5, PA6, PA7 Preamplifiers
Current Mode Preamplifiers convert the current output of a photovoltaic Ge, InAs, or InSb detector into a voltage output. They amplify the signal for subsequent use with oscilloscopes, lockin amplifiers, or AtoD converters. Three different preamp models each offer specific advantages, depending on detector type and bandwidth requirements. A comparison of preamp noise figure as a function of detector reactance is graphed in Fig. 6. All units (except multichannel models) have switchselectable gain.
The PA7 is an excellent general purpose preamplifier for most high shunt resistance (R_{D} > 25Kohm) detectors, including small area J16 Series Ge and all J16TE2 Series cooled Ge. It has extremely low current noise and current offset. For most applications, the PA770 with high gain of 10^{7} V/A offers best performance and versatility. However, for applications where 10^{7} V/A gain is unusable (due to bandwidth or DC saturation), the PA760 or PA750 are suitable alternatives.
The PA6 is a general purpose preamplifier recommended for intermediate shunt resistance (400ohm<R_{D}<50Kohm) detectors, including large area J16 Series room temperature Ge. The PA6 has very low voltage noise and offset voltage, which significantly reduces lowfrequency noise and DC drift. Standard gain settings are listed in the specification table below; custom gain settings are available.
The PA5 is recommended for low impedance detectors (R_{D}<400ohm), including J12 Series room temperature InAs and J12TE2 Series InAs. It has extremely low voltage noise and low voltage offset. However, its high current noise and current offset make it unsuitable for detectors with high impedance. Standard gain is 10^{5}, 10^{4}, and 10^{3} V/A (switchselectable). Custom gain settings are available.
Figure 5
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Model 
PA7 Series 
PA6 Series 
PA5 
Units 
PA770 
PA760 
PA750 
PA660 
PA650 
PA550 
Transimpedance Gain: (Switch Selected)  High Med Low  1E7 1E6 1E5  1E6 1E5 2.5E4  1E5 2.5E4 1E4  1E6 1E5 2.5E4  1E5 2.5E4 1E4  1E5 1E4 1E3  V/A 
Bandwidth
 nF  @ High Gain @ Med Gain @ Low Gain  8 60 150  60 1  50 200  150 200 200  60 150 200  150 200 200  200 200 200  KHz 
Input Offset Voltage (Vos) Input Bias Current (ib) Voltage Noise Density (en) @1KHz Voltage Noise from 0.1 to 10Hz Current Noise Density (in)@1KHz†  ±250 ±0.001 12 1.5 .04  ±250 ±0.001 12 1.5 .13  ±250 ±0.001 12 1.5 .04  ±100 ±12 4.5 .080 .5  ±100 ±12 4.5 .080 .64  ±80 ±30 1.1 .035 1  µV nA nV Hz^{1/2} µVpp Hz^{1/2} 
ance Maximum Output Voltage Power Requirements  < 100 ± 10 +12V and 12VDC @ 10mA  W Vpp 
Recommended for Detector Series:  J16, J16TE1, J16TE2, J16D, J10D  J16, J12TE2, J12TE3  J12, J12TE2  
PA7:4C, PA7:16C and PA7:32C MultiChannel Preamplifiers
The PA7:4C, PA7:16C and PA7:32C Series multichannel preamplifiers are designed primarily for use with Judson's NIR Array Series and XY Sensors. The preamp gain is fixed as specified at the time of purchase. Standard gain settings are 107 or 106 V/A; others are available on a custom basis. While zerovolt bias is recommended for J16P Series arrays in most applications, the preamp is also available with an optional detector bias adjust. Biasing the photodiodes improves response time and highpower linearity, but also increases dark current.
Figure 10
Model 
# of Channels 
Gain (V/A) 
Bandwidth (Max) 
PA7:4C70  4  1E7  DC to 10KHz 
PA7:16C70  16 
PA7:32C70  32 
PA7:4C60  4  1E7  DC to 60KHz 
PA7:16C60  16 
PA7:32C60  32 
PA5:4C1E3  4  1E3  DC to 200KHz

Input Offset Voltage (Vos)  ±200  µV 
Input Bias Current (ib)  ±40  pA 
Voltage Noise Density (en) @1KHz  18nVHz^{1/2} 
Voltage Noise from 0.1 to 10 Hz  2  µVpp 
Current Noise Density† in @ 1KHz  .01pAHz^{1/2} 
Output Impedance  < 100  W 
Maximum Output Voltage  ±10  Vpp 
Power Requirements PA7:4C (4 channel) PA7:16C (16 channel) PA7:32C (32 channel)  ±15 0 @ 40 @ 80  VDC ma ma ma 
Use with Detector Series:  NIR Arrays 
† At Gain = 1E7 V/A. Lower gains increase Current Noise Density. 
Voltage Mode Preamplifiers
Voltage Mode Preamplifiers may be used with photoconductive HgCdTe or with lowimpedance photovoltaics such as InAs. With photoconductive detectors, a constant bias current or constant bias voltage is applied across the detector element. The element changes resistance in response to incident photons, and the resulting change in voltage is amplified by the preamp. A blocking capacitor or DC offset circuit is required to block the constant DC bias. With photovoltaic detectors, the photocurrent generated in the detector induces a voltage across the preamp input impedance. This voltage is amplified. A lower input impedance generally results in faster frequency response, but also adds more noise to the system.
PA101 HgCdTe Preamplifier (5 Hz  1 MHz): The Model PA101 lownoise voltage preamplifier is recommended for all J15 Series HgCdTe detectors. An external bias resistor is used to set the constant bias current required for PC detector operation. When purchased with a detector, the preamp includes a bias resistor factoryselected for optimum detector performance. When ordering the preamp separately, please specify detector resistance and required bias current. The Model PA101 may also be used without bias for J12 Series InAs.
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PA8200 PbS and PbSe Preamplifier: The Model PA8200 lownoise voltage preamplifier is recommended for all J13 and J14 Series detectors. A load resistor is selected to match the detector resistance. Preamp gain and typical bandwidth specifications are listed in the table opposite. For best results, choose the preamp model with the narrowest suitable bandwidth to keep preamp noise to a minimum.
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PA300 HgCdTe Preamplifier (DC  1.0 MHz): The Model PA300 current preamplifier is designed for operation with J15D Series HgCdTe detectors. The PA300 is designed using a bridge circuit on the front end of an operational amplifier to deliver constant bias voltage across the detector. The PA300 is recommended for detectors used over a wide dynamic range in applications including FTIR's and laser monitoring. The PA300 also has a first order linearity correction in the form of a positive feedback resistor.
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Preamplifier Equivalent Circuits shown below.
Model  P/N 
Gain 
Bandwidth (Hz) 
Input Noise Voltage (nV Hz^{1/2}) 
Input Impedance (ohms) 
Max. Output (Load >1Kohm) (Vpp) 
Detector Bias 
Power Requirement 
Case Dimensions (Excluding Connectors) 
(VDC) 
(mA) 
PA101  490113  1st x100 2nd x10  10Hz to 1MHz  1.5  10K  10  Builtin  ±15  200  4.125" x 2.5" x 1.75" 
PA300  490095  100, 300, 1000  DC to 1.0MHz  1  .5  00K  10  Builtin  ±15  200  4.125" x 2.5" x 1.75" 
PA8200  6840253  12 to 300  10KHz  1.5  50K  10  External  ±15  200  2" x 3" x 1" 