One of the available choices in the ‘ design of a low-noise amplifier is the shunting of several input stages. This reduces the overall noise, Un = Uin-1/2, where un is the total noise potential of n stages; ui is the noise voltage of one stage, and n is the number of stages. This is possible because noise is a randomly composed signal. Thus, the noise signals of a number of stages at any one moment are likely to have a different frequency and phase, so that they partly neutralize one another.
In the present amplifier, three low-noise opamps, IC1-1C3, are connected in parallel. According to the manufacturers’ data sheet, the noise of a single LT1028 amount to 0.9 nV Hz-1. To this must be added the thermal -noise generated by resistors R2-R11. Circuit IC4 sums and amplifies the output signals of IC1-1C3.
Measurements on the prototype show a total noise of 0.67 nV Hz-1. According to the earlier formula, the three opamps have an overall noise of 0.52 nV Hz-i. The difference of 0.15 nV between this and the measured figure is caused by the res*isLurs, This is a low figure bearing in mind that a 1 fl resistor at room temperature generates
a thermal noise of 0.13 nV Hz–1. The amplification, a, of the circuit is computed from Alpha =
-n{1+R3/R2).R11/R8-
‘) it is necessary that resistors R3• R5, and R7: R2, R4. and R6: and R81 R9, and R10, have identical values. With values as shown, the circuit has an amplification of x600. Apart from having a low noise output, the Type LT1028 opamp is also fast: it has a slew rate of 15 V }is-1 and a bandwidth of 75 MHz for a = -1. Even when the amplification is x63, the bandwidth of the circuit as a whole, but without R12 and C17, is 1.2 MHz. However, to avoid signal overshoot, the bandwidth is limited to 500 kHz by R12-C17, which is more than adequate for even the most demanding audio application. The THD+noise ratio at a 1 kHz output at a level of 1 V is only 0.008%.
Experimenters should bear in mind that the stability of the LT1028 is internally compensated for amplifications of >x2. Since resistors R2, R4, and R6 contribute most to the overall noise, it is necessary that their value is kept as low as possible. Naturally, all resistor used should be metal film types.
With a supply voltage of ±15 V, each IC draws a current of about -7.5 mA.
