Philips AN1651 manual VI. Multiple Stage Considerations, Amplified Noise = 160μVRMS

Philips Semiconductors	Application note
Using the NE/SA5234 amplifier	AN1651

signal-to-noise ratio at the output of this stage is determined by first multiplying the gain times the signal which gives 1VRMS with a resultant noise of 400mVRMS. The signal-to-noise ratio is calculated as

￿	￿	* 4	)	+	68dB	(EQ. 5.)
S N	20log10 (1.0 4x10

S￿N 20 log		100x10* 3	￿	(EQ. 7.)
	10	￿
		1.6x10* 4

+56dB

A 56dB S/N will provide superior voice channel communications .

This is quite adequate for good quality audio applications.

Next assume that the bandwidth is cut to 3.0kHz with an input of

1mVRMS. The RMS noise is modified by the ratio of the root of the noise channel bandwidths.

	￿3x103
￿		￿@ EN + 1.6 VRMS	(EQ. 6.)
	￿20x103

Amplified Noise = 160μVRMS

	en 1.6μV
	100kΩ	en
1mVRMS	+	+
	±	±
	RS = 100Ω
	x100

x10

1mVRMS SIGNAL +1.6mVRMS NOISE

SL00636

Figure 8.

10k		+	VCC
1μF
	100k		UNITY GAIN
			VCC
	3 +		2
			1
2.2μF	2 ±
600Ω
			RL = 600Ω
1μF
10k	10k	+ VCC

				VCC
ST1700				2
	3	+	1	4.7μF
1μF	2	±
600Ω	1k			10k	100pF
40dB CIRCUIT

ST1700

DISTORTION

ANALYZER

SL00637

Figure 9. NE5234 THD Test Circuits

VI. MULTIPLE STAGE CONSIDERATIONS

Since multiple noise generators are non-coherent, their total effect is the root-of-the-sum-of-the-squares of the various noise generators at a given amplifier input.

This makes orders-of-magnitude lower noise sources less important than the higher magnitude source. Therefore, when considering the combined signal-to-noise of multiple stages of gain, the first stage in a chain dominates making its design parameters the most critical.

For this reason it is good practice to make the preamp stage gain as high as practical to boost signal levels to the second stage allowing at least an order-of-magnitude above the second-stage noise. For

instance, a signal input which exceeds the input noise of the following stage by a factor of 10:1 will only be degraded by 0.5% or -46dB, neglecting the first-stage noise. If we use the preceding example with a first-stage output signal of 100mVRMS and a 56dB S/N, and an output noise of 0.16mV. Following this with a 10kHz band limited gain-of-10 second-stage, with a 100kΩ noise source at the non-inverting input, the combined S/N is calculated as follows: (assume a 100Ω source resistance from amplifier #1)

The Second stage output noise is:

1991 Oct 6