Pioneer SX-3800 manual Excellent high frequency characteristicscompara

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I	excellent high frequency characteristicscompara-
	ble lo those of a small-signaltransistor.Further-
	more,becanselhere is no time clonstantin the NFB
I	circuit ir-rthe lorv-frecluencyregion, amplification
	is possibiedorvn to DC (DC inputs will be cut off,
	hor,vever,by the input couplingcapacitor).
r	'I'he
		circuit featuresdescribedaboveprovide an
	extremely wide polver frequency range (60W
r	+ 60W, 101-lzto ZOkHz,THD 0.005Va,Bdl).
	High SpeedBiasServocontrolCircuit
	By operating the power stageonly within the
	active region (no possiblecut-off) and with mini-
r	mr-rmidle current, the high speedbiasservocontrol
	circuit prevents the generation of switching dis-
	tortion and reducesheaf loss.
T	OperatingPrinciple
	Since idle current flows through normal classB
r	SI)PPporver stages(seeFig. 4-10) when no signal
	is appliecl,the DC level is shifted by D and VR by
	a fixed amount (w'itirthe voltageacrosspoints X
	anclY sen'ingas a bias).The voltagesacrosspoints
	X and Z, and Z		and Y at this time will be equal.
T	When the positve portion of a signalis applied to
	this circuit, the porver stagecurrent on the NPN
	side is increased,and the voltage(Vn1) acrossboth
T	ends of RB1 also heing increased,resultingin the
	voitage across point X and Z being increased.
	However, since the voltage acrosspoints X and Y
	is practically constant, the voltage across points
I	Z anrl Y (PNP power stagebias) will be decreased,
	resultingin the PNPpower stagebeingcut off.
	'I'he
		high speed bias servocontrol circuit in-
f,	creasesthe voltage acrosspoints X and Y by the
	sameamount as the voltageincreaseacrosspoints
	X and Z, thereby cancellingthe voltage decrease
	acrosspoinls Z		and	Y,	and	preventing	the PNP
	polver
F		stagefrom beingcut off.
	This high speedbias servocontrolcircuit is out-
	lined in Fig. 4-11.lVhen thereis no signalapplied
	to the crrcuit, Q1 and Q2 are almost cut off,
	while Q3 and Q4 u'illbe on. The voitage across
t	fhe collector and baseof both of thesetransistors
	(Q3 and Q4) at this time may be disregardedCon.-
	sequently, rvith the power stagebias circuit con-
I	sistingof ,trPN junctions formed by Q3, D3, and
	Q4, and VR1. this circuit is equivalent to the
	previous
	With circuil shownin Fig.4-10.
t		R1 and D1 ensuringa constantflow of cur-
	rent, the baseof Q1 and point Z may be brought
	to the samelevelon an AC basis(levelfluctuations
	due to the signal) by a simple shift in DC level.
F	Furthermore, Qf		may		be	considered emitter-
	follower lvitli R3 asthe emitter resistance.
F
F

sx-3aclo

When the voltage acrosspoints X and Z is in- creased by tl'repositive portion of the signal ap- plied to this circuit, it becomesthe input signal of this emitter-follower (Q1). Since the emitter- follower voltage gain is practically 1-, a voltage more or lessequal to that of the input signal(that is, the voltage increaseacrosspoints X and Z) is produced at R3. And the R3 voltageis the voltage applied acrossthe baseand collector of QB which forms part of the power stage bias circuit. So the bias voltage applied to Q3 will be in excessby the sameamount that the voltageacrosspoints X and

Zis increased(by positive portion of the signal) above the voltage level when no signal is being applied. Consequently, the increase in voltage acrosspoints X and Z cancelsthe decreasein volt- age across points Z and Y, thereby maintaining the idle current without cutting the PNP power stage off (noting that there actually is a slight decreasein current). For the negativeportions of the signal, Q3 and Q4 are operated in the same rnanner,thereby preventing the NPN power stage from beingcut off.

In other words, the high speedbiasservocontrol circuit acts to prevent any "poler stagecut-off"

signalsfrom beingappliedto the polyer stage.

P R E - O R I V E R

Fig .4 - 'l0 Normalpowerstagebiascircuit

P F E . O R I V E R

Fig .4 - 11 Highspeedbiasservocontrolcircuit

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