DESCRIPTION This application note describes Gennum's GM3008 Class hybr
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GM3008 Hybrid
DESCRIPTION This application note describes Gennum's GM3008 Class hybrid used typical hearing instrument application. electrical characteristics various blocks hybrid will analyzed well acoustic response curves system when used with Knowles microphones receivers. INTRODUCTION GM3008 hybrid contains Gennum's proprietary integrated circuits, GP537 preamplifier GS551 Class output amplifier. GP537 utilizes noise inverting gain blocks, each which have typical open loop voltage gain closed loop gain external resistors. Also included voltage regulator which nominal regulated output voltage 0.94 volts. regulator, which powers circuitry GP537, very good supply rejection, typically specially designed telecoil preamplifier been included, which allows manufacturer direct couple telecoil preamplifier without need coupling capacitor input. power telecoil amplifier provided external connection regulator hybrid. This provides added flexibility manufacturer allowing them forsake power telecoil circuitry should required particular application. This renders savings approximately current. GS551 integrated circuit incorporates three blocks. Block typical open loop voltage gain with closed loop external resistors. Block simply unity gain phase inverter required Class operation block Class output stage which fixed voltage gain nominally GM3008 hybrid complete Class system with required capacitors resistors board. that necessary complete high power hearing instrument addition microphone, volume control receiver. Many pins chips system have been brought give manufacturer maximum flexibility altering system parameters. With addition volume control hybrid configured gain between blocks suggested that manufacturer attempt higher than system stability will likely suffer battery impedance rises with use.
GM3008
1.3VDC GS551
GP537
REGULATOR
REFERENCE VOLTAGE
CI2960
EK3024
105k
resistors ohms, capacitors farads unless otherwise stated.
390k V.C.
Fig. GM3008 Typical Application Circuit
Document
GENNUM CORPORATION P.O. 489, Burlington, Ontario, Canada
Japan Branch: A-302, yamae age, 2-10-42 yamae, Suginami-ku Tokyo 168, Japan
tel. (905) 632-2996 fax: (905) 632-5946
tel. (03) 3334-7700 (03) 3247-8839
ELECTRICAL CHARACTERISTICS will initially analyze electrical performance system, breaking down into various blocks discussing parameters block. microphone Fig. will replaced with equivalent resistance receiver with transformer coupled load. Regulator Class systems large currents drawn from battery through load alternating fashion, half output stage conducting positive going input signal other half conducting negative portion input signal. Because this, full wave rectified signal appears supply line which proportional battery output impedance. battery ages output impedance rises rectified signal appearing supply increases. GM3008 system very high gain, critical have voltage regulator which block this supply line signal from feeding through preamplifier inputs causing instability. regulator GP537 chip nominally Power Supply Rejection Ratio (PSRR) order kHz. This means that signal appearing supply line will (approximately 5000 times) smaller amplitude regulated output pin. Fig. below displays PSRR versus Frequency regulator GP537 chip. Notice higher frequencies PSRR drops stray capacitances internal transistors becomes dominant feed through mechanism.
100k
REGULATOR VOLTAGE
SUPPLY VOLTAGE
Fig. Regulator Voltage Supply Voltage
AMPLIFIER CURRENT (mA)
SUPPLY VOLTAGE POWER SUPPLY REJECTION RATIO (dB)
Fig. Amplifier Current Supply Voltage
Notice that regulator voltage amplifier current stabilize supply voltage approximately volt. However should signal appear supply line with battery voltage volt, there will headroom regulator swing below volt before begins shut off. therefore likely that should supply voltage drop much below volts, PSRR regulator will decrease, which will cause system become unstable. GP537 regulator capable sourcing approximately before limits shuts down. Fig. illustrates response obtained when load current regulator swept from with volt supply voltage. Notice regulator voltage dropping proportionally load current increases. This effect internal resistor series with regulated output chip.
FREQUENCY (Hz)
Fig. Power Supply Rejection Ratio Frequency
regulator designed operated with minimum supply voltage approximately volts. Fig. displays regulator voltage versus supply voltage Fig. system amplifier current function supply voltage.
REGULATOR VOLTAGE
REGULATOR LOAD CURRENT (mA)
Fig. Regulator Voltage Regulator Load Current
Stage Stage GP537 noise inverting preamplifier. open loop gain system typically order closed loop gain external resistors. Normally order obtain well controlled closed loop gain wise keep closed loop gain well below that open loop. commonly suggested that closed loop gain least below open loop. GM3008, gain stage been fixed approximately with input resistor (R1) feedback resistor (R2) Figures show typical open loop voltage gain closed loop gain stage configured hybrid.
sourcing current capability preamplifier been designed nominally Process variations this parameter which dictates spread This becomes important should manufacturer wish decrease gain stage placing external resistor parallel with impedance seen output stage parallel with This calculates k//20 16.8 Therefore, assuming nominal sourcing current output stage capable 16.8 0.504 volts peak. bias this nominally 0.58 volts, therefore theoretically could swing 0.58 0.504 However this case only swing positive regulated supply voltage powering stage (nominally 0.94 volts) negative output saturation voltage (nominally 0.28 volts). limiting factor becomes saturation output stage, swing 0.36 volts positive only 0.30 volts negative before clipping occurs. Care must taken when placing external resistor parallel with that output stage swing sufficiently enough that limiting factor being able saturate stage output. also must consider that gain this stage decreased low, amount volume control adjustment range stage will limited. decrease gain stage deplete amount lower volume control resistance before reaching noise floor system. preamplifiers GM3008 have been base current compensated. they were not, current would forced flow from output input preamplifier feedback resistor satisfy input bias current requirements preamplifier. this feedback resistance happens user adjustable, case volume control, whenever feedback resistance changed audible scratching would heard current flowing resistor. With base current compensation network place there typically current flowing through feedback resistor. Stage
GM3008 STAGE VOLTAGE GAIN
13.5 -1.5
FREQUENCY (Hz)
Fig. Gain Stage GM3008 Hybrid
PREAMPLIFIER OPEN LOOP VOLTAGE GAIN (dB)
Stage virtually identical that stage open loop gain preamplifier nominally closed loop external resistors. GM3008 stage intended used volume control preamplifier which feedback resistor incorporated onto hybrid. Typically volume control would placed across pads hybrid, give maximum gain approximately around block. Lesser values volume control resistance could used however recommended that more used gain will closely approach open loop gain.
100k
Fig. Fig. portray typical open loop closed loop response stage with feedback resistor place.
FREQUENCY (Hz)
Fig. Stage Open Loop Voltage Gain
25.0 24.8 24.6 24.4 24.2 24.0 23.8 23.6 23.4
GM3008 STAGE VOLTAGE GAIN
telecoil supply (pad normally shorted regulator (pad 11), However circuit designed allow user "turn off" telecoil circuitry shorting these pads going used application, thereby saving approximately current. open loop gain telecoil nominally with closed loop value feedback resistor telecoil impedance. figures below depict open loop gain closed loop gain using resistive elements configured
TELECOIL PREAMPLIFIER CLOSED LOOP GAIN
19.8 19.6 19.4 19.2 18.8 18.6 18.4
FREQUENCY (Hz)
Fig. Gain Stage GM3008 Hybrid
PREAMPLIFIER OPEN LOOP VOLTAGE GAIN
100K
FREQUENCY (Hz)
Fig. Telecoil Closed Loop Voltage Gain
FREQUENCY (Hz)
TELECOIL PREAMPLIFIER OPEN LOOP
Fig.
Stage Open Loop Voltage Gain
VOLTAGE GAIN (dB)
Stage GP537 chip also designed with nominal sourcing current bias output 0.58 volts. maximum swing +0.36 volts Vreg, -0.30 volts Vsaturation. Telecoil telecoil amplifier GP537 chip designed allow manufacturer direct couple telecoil onto preamplifier without addition external coupling capacitor. telecoil circuit consists distinct parts preamplifier reference voltage. output voltage reference circuit (pad precisely matches that telecoil input bias voltage (pad 14). idea being that when telecoil placed from reference voltage preamp input, current will flow through coil potential both ends identical.
100K
FREQUENCY (Hz)
Fig. Telecoil Open Loop Voltage Gain
Stage Stage GS551 much like preamplifiers GP537. open loop gain also typically closed loop ratio external resistors. GM3008 hybrid closed loop gain been Fig. illustrates closed loop gain block configured hybrid. open loop difficult measure device internally loop closed with connections stage
wise reference voltage appears ground. When inductively induced signal injected into coil current will flow into preamplifier cause amplified voltage appear output.
GM3008 STAGE VOLTAGE GAIN (dB)
Maximum Power Output inserting resistor series with receiver power supply line, possible limit maximum output level Maximum Power Output (MPO) system. Effectively, limiting voltage that stage drive receiver dropping voltage across resistor. Fig. illustrates electrical action GM3008 hybrid.
RMPO=0
OUTPUT (dBV)
RMPO=100 RMPO=500 -100 RMPO=1k
FREQUENCY (Hz)
Fig. Voltage Gain Stage GM3008 Hybrid
current sourcing capability this stage nominally However, this point (pad sees impedance into stage parallel with very high impedance into stage parallel with hybrid. Neglecting impedance into stage this calculates 8.57 This means that with current source hybrid theoretically swing 8.57 0.43 volts peak. manufacturer wishes decrease gain this stage placing resistor parallel with must remembered decrease much that stage becomes limiting element system. other words, voltage begins clip prior stage output reaching maximum level. Stage Stage GS551 unity gain inverting stage. input feedback resistors this stage both rendering unity gain, inverting amplifier. This stage required class operation stage
INPUT (dBV)
Fig. Maximum Power Output RMPO
Tone Control Should manufacturer wish tone control network system, there couple ways could accomplished. would network immediately following microphone. Fig. shows this might look connected hybrid. basic operation this network follows. Initially shall assume that maximum resistance (100 frequencies impedance capacitor will very high,in series with Therefore impedance ground will very high attenuation input signal will basically frequency increases reactance capacitor will decrease, however still attenuation input signal will minimal.
Stage Stage efficient class amplifier capable delivering greater than 50mA current load. typical voltage gain this stage fixed
GM3008 STAGE VOLTAGE GAIN (dB)
REGULATOR
105k
100k
resistors ohms, capacitors farads unless otherwise stated
FREQUENCY (Hz)
Fig. Voltage Gain Stages GM3008 Hybrid
Fig. Tone Control Network.
GM3008 STAGE GAIN (dB)
Assume that been decreased corner frequency network would calculated using equation: (2RMICCT) Please note that this equation only very crude estimation actual transfer function. were actually include effects hybrid calculation would become more complicated. Fig. measured electrical frequency response hybrid, with this tone control network place.
RTC=100k 12.5 RTC=0 -2.5
FREQUENCY (Hz)
Fig. Effect Tone Control Circuit Stage Gain
Acoustic Measurements FONIX 6500 printouts below illustrate acoustic response GM3008 hybrid with variety series receivers with EK3024 microphone.
2.21
Fig. GM3008 with EK3024 CI2960
3.80
Fig. GM3008 with EK3024 CI2748
3.96
Fig. GM3008 with EK3024 CI2746
5.29
Fig. GM3008 with EK3024 CI2955
CI2955 CI2746 CI2960 CI2748 =100k CI2960
SYSTEM GAIN (dB)
SYSTEM GAIN (dB)
FREQUENCY (Hz)
FREQUENCY (Hz)
Fig. GM3008 with EK3024 Receivers dBSPL Input)
Fig. Effects Tone Control Circuit Acoustic Gain
Tone Control tone control circuit which previously measured electrically, measured acoustically using EK3024 microphone CI2960 receiver. gain (Reference Test Gain) input SPL. Fig. depicts results. more high frequency larger value capacitor could used however caution should taken large value will then significantly attenuate input even maximum resistance.
ACOUSTIC Once again with hybrid configured Fig. value resistor series with receiver supply line varies results noted FONIX printouts.
2.39
Fig. GM3008 with EK3024 CI2960 REC, RMPO
1.61
Fig. GM3008 with EK3024 CI2960 REC, RMPO
1.09
Fig. GM3008 with EK3024 CI2960 REC, RMPO
Gennum Corporation assumes responsibility circuits described herein makes representations that they free from patent infringement. Copyright April 1993 Gennum Corporation. rights reserved. Printed Canada.
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