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Volume 92, Number 6, November-December 1987 Journal of Research of the National Bureau of Standards

A Low Noise Cascode Amplifier

Volume 92 Number 6 November-December 1987

Steven R. Jefferts

Joint Institute for Laboratory Astrophysics

We describe the design, schematics, and performance of a very low noise FET cascode input amplifier. This ampli-

Key words: cascode amplifier; low bias current amplifier; low noise FET ampli­fier; noise analysis; noise current; noise voltage.

University of Colorado and National Bureau of Standards Boulder, CO 80309

and

fier has noise performance of less than 1.2 nV IVHz and 0.25 fA;VHz over the 500 Hz to 50 kHz frequency range. The amplifier is presently being used in con­junction with a Penning ion trap but is applicable to a wide variety of uses requiring low noise gain in the I Hz to 30 MHz frequency range,

F. L. Walls National Bureau of Standards Boulder, CO 80303

Introduction

A low noise amplifier has been designed using a 2SKI17 N channel J-FET as the input device in a cascode [1] configuration. Noise measurements on this amplifier yield a low frequency noise cur­rent of 0.25 fA/YHz and a voltage noise of less than 1.2 nV /YHz in the 500 Hz to 50 kHz region. Bloyet et al. [2] suggest a figure of merit of the product of the noise voltage and current as being appropriate for amplifiers of this type. This ampli­fier has a figure of merit of - 3 X lO-H W 1Hz, which is almost two orders of magnitude smaller than other amplifiers reported elsewhere. [2]

The amplifier described here is presently being used in conjunction with a Penning trap to detect small image currents ( -0.01 pA) induced by ion motion in the trap. [3] This amplifier also appears to be well suited for use in noise thermometry ex­periments. [4]

383

Accepted: June 8, 1987

This paper discusses some general design criteria for cascade amplifiers and draws some conclusions concerning the optimum choice of FETs for such amplifiers. A particular design having the noise performance described above is presented and ana­Iyzed. Variations of the design which either have much larger bandwidth, 30 MHz, or draw ex­tremely low input bias current, less than 0.01 pA, are briefly discussed.

Equivalent Circuit for Noise Analysis

The schematic of the amplifier is shown in figure 1. The biasing scheme used for Q2, the com­mon base portion of the cascode, is attractive for its simplicity and inherent low noise. However, to work properly it requires that Idss [5] of Q2 be larger than I dss of Q I .

Volume 92, Number 6, November-December 1987 Journal of Research of the National Bureau of Standards

Ikn INPUT

(!;'--....-

soon soon v+

'* IOI' F iI2-30V)

IMn OP-27180I'F

'----+ OUTPUT

19kn :t 2kn

200F

'------.w..-_I I kn Rs IMn

Fillllre 1. Schematic diagram of the low noise preamplifier. NOTE: I) All resistors I % metal film. 2) All capacitors are tantalum. 3) V + must be well filtered. 4) The OP-27 power supply leads should

be bypassed with 10 nand 0.1 F close to the OP-AMP.

The gain of the cascode input stage is large, about 50. Hence, the noise in this stage is the domi­nant noise mechanism in the amplifier, and we will therefore confine our analysis to the cascode input stage and the associated biasing circuitry. The sig­nal frequency equivalent circuit of the input stage is illustrated in figure 2. From figure 2 we can pro­ceed to draw the noise equivalent circuit as shown in figure 3. Using this model, we can write the equivalent input noise, En;, as [6]

2 2 2 2 Z2 ( I ) 2 E ni = E nRg + E nQI +

1 nQ! g + Ko!

where Kt = gmlRd and KQ! = -gm1/gm2 is the gain of the common source component of the cas­code. Zg is the impedance presented to the gate of Q 1 formed by the parallel combination of Cg, the gate capacitance, and Rg, the gate bias resistor. The choice of Q2 is governed by a tradeoff between

OUTPUT

INPUT

10n

Fillllre 2. Signal frequency equivalent circuit of the cascode inp ut stage.

384

bootstrapping Cgd of Ql for lowest input capaci­tance and gain in Q 1 suppressing voltage noise in Q2 relative to Q1. This suggests that the choice of identical FETs for Ql and Q2 may not be opti­mum. For this amplifier, we chose Q2 to be a 2N4416, yielding gm1/gm2 - 4, which suppresses the voltage noise of Q2 well below that of Q 1 and still provides a reduction of input capacitance from -44 to -11 pF.

Noise Measurements

The amplifier noise was determined by first mea­suring the transfer function of the amplifier on a spectrum analyzer (see fig. 4). The input capaci­tance was then obtained by using a known value of the capacitor in series with the input of the ampli­fier and measuring the change in apparent amplifier gain as a function of capacitance. In order to mea­sure the input current noise, the gate bias. resistor, Rg, was increased to 7 X lOll n so that the term I;Q! zi would dominate in eq (I ). A measurement of the noise from 1.5 to 10 Hz coupled with the known input capacitance, Cg, allows one to write

(2)

where Eni (j) is the equivalent noise at frequency / at the input of Q1. Using a linear regression analy­sis to find the slope, m, of the 1/ En; vs / line, we can then write

1 _ 21TCg nQ! - m (3)

This analysis holds, assuming that the thermal noise current of Rg does not swamp the noise current of QI and that the perturbation of 1// noise is small. The first assumption is easily checked as our

Volume 92, Number 6, November-December 1987 Journal of Research of the National Bureau of Standards

gm2 VGS2 OUTPUT - D2

EnQI 52

INPUT

gm1 InQ2

VGS1

EnQ2 G2

Figure 3. The noise equivalent circuit of the cascode input state.

7X lO" n resistor used for Rg generates only 0.15 fA/VHz noise current which is of the same order of magnitude as the noise current associated with Q1. The 1// contribution of current noise in both the input FET and Rg was measured to be less than 10-16 A/VHz at 1.5 Hz.

1000

I z -<[ (.!)

lOO

10 10 102 103 104 105

F REQUENCY (Hz)

Figure 4. Measured transfer function of the amplifiers.

EnQI> the 'voltage noise associated with QI, was measured by replacing the 7 X 10" n resistor used for Rg with a 10 n resistor. This makes the IQI Zi term in eq (1) insignificant and the equation can then be rewritten to yield:

385

If the noise associated with Q2, gm1/gm2, and the output noise are measured, one can infer the noise associated with Q I.

Results

Measurements using three different 2SK 117 FETs for QI and a variety of different 2N4116 FETs for Q2 give the following results for the amplifier

En; = I . I nV /VHz In; = 0.25 fA/VHz . (5)

Figure 5 shows the measured voltage noise as a function of frequency for the amplifier. Indepen­dent measurements with 2N4416 FETs show that the noise voltaassociated with them is approxi­mately 3 nV /VHz. Using this value and eq (5) we can infer a noise voltage for the 2SK 117 of about 0.8 nV /VHz. It is interesting to compare this to the theoretical result derived by van der Ziel: [7]

_ (4KT) 1/2 enQI - 3 gm! . (6)

Using gm! I 60 n' the transconductance of the

Volume 92, Number 6, November-December 1987 Journal of Research of the National Bureau of Standards

2SK 117 at 3 mA drain current, we obtain enQI = 0.82 nV Iv'Hz which is in agreement (possi­bly fortuitous) with the measured result.

...... > .5 W UI 0 Z W C) Cl I-..J 0 >

100

10

10 102 103 FREQUENCY (Hz'

104

Figure 5. Measured input voltage noise.

..... Cl

10 !:. w 0 Z

I I-Z W It: a: :::> u

If one measures the gate current of the input FET in a version of this amplifier in which Q2, the common gate portion of the cascode, is shorted, making the input of the amplifier a common source stage, an interesting effect occurs. The gate cur­rent, as measured by the voltage drop across Rg, decreases and finally changes sign with increasing drain current. A measurement of the noise current in this region suggests that in fact two (at least) competing currents are responsible, as the noise current is monotonically increasing in the region of apparently zero gate bias current. This is as would be expected for the noise from two competing pro­cesses. Thus, this effect is potentially useful in an application in which the amplifier must draw a minimal bias current through the gate. However, a drawback to this circuit is that the input capaci­tance is - 50 pF as opposed to - 11 pF for the cas­code configuration. The cascode amplifier also exhibits very low input bias current, typically less than 0.3 pA for drain currents in the 3 mA range, but it does not exhibit an apparent vanishing of this bias current as does the common source configura­tion. It should be noted that this effect prevents us from inferring that the noise current in Ql is due to shot noise in the measured gate current ofQl, since the true gate current is not a well determined quan­tity in the presence of these competing currents.

The bandwidth of the amplifier as shown in figure 1 is limited to about 500 kHz. This band­width limitation is, however, due to the limited bandwidth of the op-amp used for the output stage. If additional bandwidth is required, Rc and Rd should be reduced and a video amplifier should be used as the output stage.

386

Conclusion

We have discussed the design and test of a FET cascode input amplifier with extremely low voltage noise, less than 1.2 nV Iv'Hz, and ex­tremely low current noise, 0.25 fA/v'Hz.

This amplifier also has a low input capacitance of II pF. Thus it can be used to provide useful low noise gain from I Hz to more than 30 MHz. An­other significant attribute is the very low bias cur­rent drawn by the amplifier, less than 0.3 pA; a modified version of this amplifier draws even less input bias current. A short discussion of design criteria and noise mechanisms in cascode amplifiers is also provided.

Acknowledgments

The authors are grateful to John Hall and David Howe for their many useful comments and sugges­tions. Steven R. Jefferts would also like to thank Gordon Dunn for his support and encouragement on this project.

About the Authors: Steven R. Jefferts is with the Joint Institute for Laboratory Astrophysics University of Colorado and the National Bureau of Standards. F. L. Walls is a physicist with the Time and Fre­quency Division in the NBS Center for Basic Stan­dards.

References

[I) The term cascode amplifier refers, in a historical sense, to a

pair of triode vacuum tubes operated as a grounded cathode amplification stage followed by a grounded grid amplification stage. In the case of bipolar transistors it

refers to a common emitter stage followed by a common

base stage and in FETs it is a common source-common gate pair as used here. Hybrid cascode amplifiers using a common source FET followed by a common base bipolar

transistor are also common. See, for example, R. Q. Twiss and Y. Beers, in Vacuum Tube Amplifiers, MIT Radiation LABS Series edited by G. E. Valley Jr. and M. Wall man (Boston Technical Lithographers, 1963), Ch. 13.

(2) Bloyet, D., Lepaisant, J., and Varoquaux, E., Rev. Sci. In­strum. 56, 1763 (1985).

(3) Barlow, S. E., Luine, J. A., and Dunn, G. H., Int. J. Mass.

Spectrom. Ion Proc. 74, 97 (1986).

(4) Anderson, P. T., and Pipes, P. B., Rev. Sci. Instrum. 45, 42 (1974).

(5) ldss is the value of the saturated drain-source current in a

FET operated at zero gate source voltage. (6) Motchenbacher, C. D., and Fitchen, F. c., in Low Noise

Electrical Design (Wiley, New York, 1973), p. 231. (7) van der Ziel, A., Proc. IEEE 50, 1808 (1962).

2N4416A2N4416A

Document Number 3631

Issue 1

Semelab plc. Telephone +44(0)1455 556565. Fax +44(0)1455 552612.

E-mail: sales@semelab.co.uk Website: http://www.semelab.co.uk

Semelab Plc reserves the right to change test con-ditions, parameter limits and package dimensions without notice. Information furnished by Semelab is believed to be both accurate and reliable at the time of

going to press. However Semelab assumes no responsibility for any errors or omissions discovered in its use. Semelab encourages customers to verify that

Semelab plc. Telephone +44(0)1455 556565. Fax +44(0)1455 552612.

E-mail: sales@semelab.co.uk Website: http://www.semelab.co.uk

10mA

300mW

2.4mW / °C

–55 to 150°C

–55 to 200°C

SMALL SIGNAL

N–CHANNEL J–FET THAT IS

DESIGNED TO PROVIDE HIGH

PERFORMANCE AMPLIFICATION AT

HIGH FREQUENCIES

FEATURES

• EXCELLENT HIGH FREQUENCY GAINS

• CECC SCREENING OPTIONS

• SPACE QUALITY LEVEL OPTIONS

APPLICATIONS:

The 2N4416 and 2N4416A are N-ChannelJFETs designed to provide high-performanceamplification, especially at high-frequency.

VGD Gate – Drain Voltage

VGS Gate – Source Voltage

IG Gate Current

PD Power Dissipation

Derate

Tj Operating Junction Temperature Range

Tstg Storage Temperature Range

–30V

–30V

–35V

–35V

MECHANICAL DATA

Dimensions in mm (inches)

TO-72(TO-206AF)

ABSOLUTE MAXIMUM RATINGS

(Tamb = 25°C unless otherwise stated)

PIN 1 - Case

PIN 3 -Drain

PIN 2 - Gate

PIN 4 - Source

0.48 (0.019)0.41 (0.016)

dia.

4.95 (0.195)4.52 (0.178)

4.95 (0.195)4.52 (0.178)

5.3

3 (

0.2

10)

4.3

2 (

0.1

70)

12.7

(0.5

00)

min

.

1

2

3

4

2.54 (0.100)Nom.

2N4416 2N4416A

2N4416A2N4416A

Document Number 3631

Issue 1

Semelab plc. Telephone +44(0)1455 556565. Fax +44(0)1455 552612.

E-mail: sales@semelab.co.uk Website: http://www.semelab.co.uk

Semelab Plc reserves the right to change test conditions, parameter limits and package dimensions without notice. Information furnished by Semelab is believed

to be both accurate and reliable at the time of going to press. However Semelab assumes no responsibility for any errors or omissions discovered in its use.Semelab encourages customers to verify that datasheets are current before placing orders.

Parameter Test Conditions Min. Typ. Max. Unit

ELECTRICAL CHARACTERISTICS (TA= 25°C unless otherwise stated)

VDS = 0V 2N4416

IG = –1µA 2N4416A

VDS = 15V 2N4416

ID = 1nA 2N4416A

VDS = 15V VGS = 0V

VGS = –20 VDS = 0V

Tamb = 125°C

VDG = 10V ID = 1mA

VDS = 10V VGS = –10V

IG = 1mA VDS = 0V

VGS = 0V ID = 1mA

VDS = 15V VGS = 0V

f = 1kHz

VDS = 15V VGS = 0V

f = 1MHz

VDS = 10V VGS = 0V

f = 1kHz

V(BR)GSS Gate – Source Breakdown Voltage

VGSS(off) Gate – Source Cut–off Voltage

IDSS* Saturation Current

IGSS Gate Reverse Current

IG Gate Operating Current

ID(off) Drain Cut–off Current

VGS(F) Gate – Source Forward Voltage

RDS(on) Drain – Source On Resistance

gfs Common – Source Forward

Transconductance

gos Common – Source Output

Transconductance

Ciss Common – Source Input Capacitance

Common – Source Reverse TransferCrss

Capacitance

Common – Source OutputCoss

Capacitance

_en Equivalent Input Noise Voltage

–30 –36

–35 –36

–3 –6

–2.5 –3 –6

5 10 15

–2 –100

–4 –100

–20

2

0.7

150

4.5 6 7.5

15 50

2.2 4

0.7 0.8

1 2

6

V

mA

pA

nA

pA

V

Ω

ms

µs

pF

nV √Hz

STATIC CHARACTERISTICS

DYNAMIC CHARACTERISTICS

Pulse Test; PW = 300µs, Duty Cycle # 3%

2SK117

2003-03-25 1

TOSHIBA Field Effect Transistor Silicon N Channel Junction Type

2SK117

Low Noise Audio Amplifier Applications

• High |Yfs|: |Yfs| = 15 mS (typ.) (VDS = 10 V, VGS = 0)

• High breakdown voltage: VGDS = −50 V

• Low noise: NF = 1.0dB (typ.)

(VDS = 10 V, ID = 0.5 mA, f = 1 kHz, RG = 1 k)

• High input impedance: IGSS = −1 nA (max) (VGS = −30 V)

Maximum Ratings (Ta ==== 25°C)

Characteristics Symbol Rating Unit

Gate-drain voltage VGDS −50 V

Gate current IG 10 mA

Drain power dissipation PD 300 mW

Junction temperature Tj 125 °C

Storage temperature range Tstg −55~125 °C

Electrical Characteristics (Ta ==== 25°C)

Characteristics Symbol Test Condition Min Typ. Max Unit

Gate cut-off current IGSS VGS = −30 V, VDS = 0 −1.0 nA

Gate-drain breakdown voltage V (BR) GDS VDS = 0, IG = −100 µA −50 V

Drain current IDSS

(Note) VDS = 10 V, VGS = 0 1.2 14 mA

Gate-source cut-off voltage VGS (OFF) VDS = 10 V, ID = 0.1 µA −0.2 −1.5 V

Forward transfer admittance Yfs VDS = 10 V, VGS = 0, f = 1 kHz 4.0 15 mS

Input capacitance Ciss VDS = 10 V, VGS = 0, f = 1 MHz 13 pF

Reverse transfer capacitance Crss VGD = −10 V, ID = 0, f = 1 MHz 3 pF

NF (1) VDS = 10 V, RG = 1 kΩ

ID = 0.5 mA, f = 10 Hz 5 10

Noise figure

NF (2) VDS = 10 V, RG = 1 kΩ

ID = 0.5 mA, f = 1 kHz 1 2

dB

Note: IDSS classification Y: 1.2~3.0 mA, GR: 2.6~6.5 mA, BL: 6~14 mA

Unit: mm

JEDEC TO-92

JEITA SC-43

TOSHIBA 2-5F1D

Weight: 0.21 g (typ.)

2SK117

2003-03-25 2

2SK117

2003-03-25 3