Low power audio amplifier MC34119-03

The internal circuit block diagram of MC34119 is shown in Figure 5-43. Its main circuit consists of two operational amplifiers 1 and 2. Amplifier
1 has an open-loop gain greater than 80dB when the signal frequency is less than 100Hz. Its use is similar to an operational amplifier, and
main amplifiers with different gains can be formed through external feedback resistors. It can also be connected to inverting or non-inverting input mode. The non-inverting input
can have higher input impedance (125kfl). Amplifier 2 is connected as an inverter, with a gain of -10. The speaker is connected to the output terminals of op amps l and
2 (between pins ⑤ and ⑥), so this is a single-ended audio input with A and double-ended differential output. Amplifier Gan Figure 5-43
In the circuit, op amp 1 is connected in the inverting input mode, and its own gain is determined by the resistor support and wind as Rc/R, and for the speaker differential
output end, the total gain of the circuit is doubled, which is 2RIlR . Since the unit gain bandwidth of op amps 1 and 2 is about 1.5MHz,
in order to make the closed-frequency response cover the entire telephone voice band (300Hz~3400Hz) during closed-loop operation, the closed-loop gain
should not . That is Rr/R． Should be ≤1 00.
    The MC34119 can provide a maximum output current of ±200mA to the speaker. When outputting the maximum current, the two output terminals
can reach a minimum of 0.4V when the power supply voltage is greater than 6V. When the output current is less than 200mA, the output swing range will be
larger.
    In static state, the DC output offset voltage of the amplifier is yo. -V has little to do with the input offset voltage of the operational amplifier inside the device
, because the input A offset voltages of operational amplifier 1 and operational amplifier 2 are usually close, so they can almost cancel each other out at the output end
. And V. , l—y. , the value of 2 is proportional to the feedback resistor damping and has nothing to do with the size of the power supply voltage. 6 As can be seen from Figure 5-43, since
the input bias current /IB of the op amp l flows out of the op amp. And flows through the ④ pin (y swelling) through the external connection to the audio amplifier output terminal ⑤
pin (yo,) into the operational amplifier 1, which makes v. Relative input negative change R.. lIB, while the voz meridian reverses the phase and changes by the same value in the positive direction,
Lang V. ,, a mine. ,2-12Rf-IJB. Since the IJB of the device has nothing to do with the power supply voltage, changes in the power supply
have no impact on the output offset voltage of the amplifier. Considering the static offset voltage at both ends of the small speaker, the Rf resistance should not be too large. Since the input bias current of MC34119
is very small, it generally does not exceed 200nA (typical value is 100nA). When RI=lOkfl, its vo. One Voz will not exceed
4m V, so the impact is not significant.
    The ④ pin (FCl) and @ pin (FC2) of MC34 can be connected to the ground with appropriate external capacitors to improve the power supply rejection
capability of the device (PSRR, which refers to the rate of change of the amplifier's input offset voltage with the power supply voltage) as shown in the figure Cl. C2. Cl. C2 in 5-43
The larger the value, the stronger the power supply suppression capability of the amplifier, but the increase in capacitance will also affect the turn-on time of the circuit when it is powered on,
because these two capacitors must pass Internal 50kfl and 1 2:5kil roar resistance charging. When C． - 1S,;F, Cz-5LF, when the power supply is 6V,
the circuit turn-on time takes about 100 ms. If the circuit is powered by a regulated power supply, Cl. C2 can be smaller; connect the SpLF capacitor to ② pin (FC2)
. When the luF capacitor is connected to pin ③ (FCl), it can provide a power supply rejection ratio of approximately 52dB. If the capacitor on pin ③ FC1 is
large enough, there is no need to connect the capacitor to pin ② FC2. If the circuit relies on telephone line signals to drive the speaker, better filtering is required, that is,
larger values ​​of C1 (5juF) and C2 (10VF) are needed.