Amplifier Noise

Noise of a Degenerated LTP Consider an LTP input stage biased with a tail current of 1 mA and degenerated with

470- W emitter resistors. Assume that the stage is fed from a voltage source. The noise contributions of the transistors and degeneration resistors will each be increased by a

factor of 2 because there are two of each in series. The resistor noise of each emitter resistor is 29 . nV/ Hz . The collector shot noise current of each transistor will be

. pA/ Hz 12 7 . Transistor gm is 19.3 mS. Input-referred base-emitter noise is . 0 66 nV/ Hz . Input-referred voltage noise of each half of the LTP is thus 30 . nV/ Hz , with the degeneration resistor noise strongly dominating. Input

voltage noise for the stage is 3 dB higher, at 42 . nV/ Hz .

Now assume that the stage is fed from a 1-k W source. Resistor noise is 41 . nV/ Hz . Assume that transistor beta is 100. Base current is 5 µA. Input noise current is

13 . pA/ Hz . Input noise voltage due to input noise current is 13 . nV/ Hz . Total input

noise voltage across the input impedance is thus 43 . nV/ Hz .

Total input noise for the arrangement is the power sum of 42 . nV/ Hz and

43 . nV/ Hz , which is 60 . nV/ Hz .

JFET Noise JFET noise results primarily from thermal channel noise. That noise is modeled as an

equivalent input resistor r n whose resistance is equal to approximately 0.6/gm [7]. If we model the effect of gm as rs’ (analogous to re’ for a BJT), we have r n = 0.6rs’. This is remarkably similar to the equivalent voltage noise source for a BJT, which is the voltage noise of a resistor whose value is re’/2. The noise of a BJT goes down as the square root

of I c because gm is proportional to I c , and re’ goes down linearly as well. However, the gm of a JFET increases as the square root of I d . As a result, JFET input voltage noise goes down as the 1/4 power of I d . The factor 0.6 is approximate, and SPICE modeling of the LS844 suggests that the number is closer to 0.67. At I d = 0.5 mA, gm for the LS844 is about 1 mS, corresponding to a resistance rs’ of 1k W. Multiplying by the factor 0.67, we have an equivalent resistance r n of 670 W, which

has a noise voltage of 34 . nV/ Hz .

Input and VAS Circuits

153

Noise Simulation With an understanding of the basics of noise and the cause-effect relationships, noise

analysis is best handled by SPICE simulations. In this approach, the noise contribution of every element can be evaluated by clicking on the circuit element. The base-current noise in a simulation will show up as a component of the voltage noise in the resistors that make up the source impedance to the base node.

References

1. Otala, M., “Transient Distortion in Transistorized Audio Power Amplifiers,” IEEE Transactions on Audio and Electro-acoustics, vol. AU-18, pp. 234–239, September 1970.

2. Leach, W. M., “Transient IM Distortion in Power Amplifiers,” Audio, vol. 59, no. 2, pp. 34–41, February 1975.

3. Jung, W. G., Stephens, M. L., and Todd, C. C. “Slewing Induced Distortion and Its Effect on Audio Amplifier Performance–With Correlated Measurement Listening Results,” AES preprint No. 1252, presented at the 57th AES Convention, Los Angeles, May 1977.

4. Cordell, R. R., “Another View of TIM,” Audio, pp. 38–49 February, & pp. 39–42 March, 1980; available at www.cordellaudio.com.

5. “The Apt 1 Power Amplifier Owner’s Manual,” Apt Corporation, 1979.

6. Cordell, R. R., “A MOSFET Power Amplifier with Error Correction,” Journal of the Audio Engineering Society, vol. 32, no. 1, pp. 2–17, January 1984; available at www .cordellaudio.com.

7. Haslett, J. W., and Trofimenkoff, F. N. “Thermal Noise in Field-effect Devices,” Proceedings of the ROC. IEE, vol. 116, no. 11, pp. 1863–1868, November 1969.