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I | excellent high frequency characteristicscompara- | ||||||
ble lo those of a | |||||||
more,becanselhere is no time clonstantin the NFB | |||||||
I | circuit | ||||||
is possibiedorvn to DC (DC inputs will be cut off, | |||||||
hor,vever,by the input couplingcapacitor). | |||||||
r | 'I'he |
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| circuit featuresdescribedaboveprovide an | ||||||
extremely wide polver frequency range (60W | |||||||
r | + 60W, | ||||||
High SpeedBiasServocontrolCircuit |
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By operating the power stageonly within the | |||||||
active region (no | |||||||
r | |||||||
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| circuit prevents the generation of switching dis- | ||||||
| tortion and reducesheaf loss. |
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T | OperatingPrinciple |
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Since idle current flows through normal classB | |||||||
r | SI)PPporver stages(seeFig. | ||||||
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 |
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| 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 |
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F |
| stagefrom beingcut off. |
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This high speedbias servocontrolcircuit is out- | |||||||
lined in Fig. | |||||||
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 |
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With circuil shownin |
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| 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 |
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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
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
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
I J