Philips AN1651 Using the NE/SA5234 amplifier, Intermediate Amplifier and Output Stage Figure

Philips Semiconductors	Application note
Using the NE/SA5234 amplifier	AN1651

VCC	VIN	VCC		VIN	V
	VOUT			VOUT		CC
5V		47k				5V
		+
		VIN
VGND	t	±	VOUT	t	VGND
		47k
			VGND
	CONVENTIONAL OP AMP			PHILIPS NE5234	SL00569

Figure 2. Output Inversion Protection

(VOLTS)	6
	5
VOLTAGE
	4		ªN-MODEº		ªLARGE
MODE	3		CMRR		SIGNALº
			VEE+1 < VCM < VCC		CMRR
COMMON	2
	1		ªN-MODEº
0.5
		VEE < VCM < VEE+0.5V		CMRR
	-0.1	+0.1	VOS		VEE
	-1
			mV

NE5234 Common-Mode Operating Regions SL00631

Figure 3.

For negative going input signals, which drive the inputs toward the VEE rail and below, another set of diode-connected transistors come into operation. These steer the current from the input into Q8 or Q9 emitter circuits again preventing the reversal effect.

Figure 3 shows graphically how the N and P mode transitions relate to the common-mode input voltage and the offset voltage VOS.

Intermediate Amplifier and Output Stage (Figure 4)

The intermediate stage is isolated from the input amplifier by emitter followers

to prevent any adverse loading effect. This stage adds gain to the over all amplifier and translates levels for the following class-AB current-control driver. Note that I2 is the inverting input and I1 the

non-inverting input. The output is taken from multiple collectors on the non-inverting side and provides matching for the following stage.

Class-AB control of the output stage is achieved by Q61 and Q62 with the associated output current regulators. These act to monitor the smallest current of the non-load supporting output transistor to keep it in conduction. Thus, neither Q71 or Q81 is allowed to cutoff but is forced to remain in the proper Class-AB region.

Overload protection is provided by monitor circuits consisting of R76-D2 for sinking and R86-D3 for sourcing condition at the output. When the output current, source or sink, reaches 15 milliamperes, drive current to the stage is shunted away from current sources IB6 or IB9 reducing base current to driver transistors Q72 and Q82 respectively.

The prevention of saturation in the output stage is achieved by saturation detectors Q78 and Q88. When either Q71 or Q81 approaches saturation, current is shunted away from the driver transistors, Q72 or Q83 respectively.

III. CHARACTERISTICS

Internal Frequency Compensation

The use of nested Miller capacitors C2 through C6, in the intermediate and output sections, provides the overall frequency compensation for the amplifier. The dominant pole setting capacitor, C2, provides a constant 6dB/octave roll-off to below the unity gain frequency of 2.5MHz. Figure 5 shows the measured frequency response plot for various values of closed-loop gains.

1991 Oct 3