Discrete low noise, low distortion preamplifier

Discrete low noise, low distortion preamplifier

Function of the circuit

OP amplifiers developed for audio equipment include NE5532A and LM833A, which are known for their low noise, but these ICs are restricted by input impedance, high-frequency characteristics, and power supply voltage. Transistor circuits using discrete components have the freedom to design according to usage requirements.

Since the input stage uses a low-noise parallel J-FET circuit, low-noise characteristics can be obtained even in circuits with high signal source output resistance. For bipolar input low-noise OP amplifiers, the noise factor decreases as the signal resistance decreases.

How the Circuit Works

The input stage is a constant current biased FET composed of TT7 and sent to the amplifier circuit. Each group of 3 FETs is connected in parallel to achieve low noise. In the J-FET circuit, the noise characteristics will not change greatly due to the bias current, so the distortion value can be reduced. The drain resistance RD (i.e. R2, R3) is related to the voltage gain of this stage (AV≈GM.RD). When a larger open-loop gain is required, the drain resistance can be increased or an active load circuit can be added.

In the differential amplifier circuit composed of TTB.D, diode D1 is used for temperature compensation of V. Because the voltage gain of the entire circuit is basically determined by this stage circuit, a current Miller circuit is added to convert the differential output into a unipolar one.

The output stage is a push-pull emitter follower, with base bias generated by diodes D2 and D3. Therefore, it has driving capability even under low load impedance conditions and can reduce waveform distortion.

Component Selection

In a multi-pole amplifier circuit, the noise characteristics of the primary determine the S/N of the entire circuit, so low-noise components should be selected. TT1~TT0 uses low-noise FETs (2SK68A) with large mutual conductance GM.

Metal resistors can be used as the peripheral resistors of FET, rather than carbon film resistors or solid resistors. C1 determines the low-frequency characteristics of the circuit. The lower the frequency, the larger the required capacity. If possible, tantalum capacitors should be used. In the case where the voltage gain requirement is not high, R6 can be directly grounded. Since the output stage is a push-pull circuit, it can also be driven under the condition of small load resistance, so TT12 and TT13 should choose transistors with large collector rated power consumption. The emitter resistors R10 and R11 are about 1/10 of the load resistance.

Adjustment

Since the bias current of the input stage determines the DC operating point of the next stage, the source resistor R4 in the FET constant current circuit must be adjusted. The resistor should be around 1 kilo-ohm. If the adjustment is troublesome, it can be replaced with a semi-fixed resistor of 2 kilo-ohm.

If C1 is removed and DC coupling is used, the compensation of the differential amplifier should be adjusted, a 100-ohm variable resistor should be connected between the sources of the two FETs, and the sliding contact should be connected to the drain of TT7.