The LTC®6261/LTC6262/LTC6263 are single/dual/quad operational amplifiers with low noise, low power, low supply voltage, and rail-to-rail inputs and outputs. They are unity gain stable with capacitive loads up to 1nF. They feature 30MHz gain-bandwidth product, 7V/μs slew rate while consuming only 240μA of supply current per amplifier operating on supply voltages ranging from 1.8V to 5.25V. The combination of low supply current, low supply voltage, high gain bandwidth product and low noise makes the LTC6261 family unique among rail-to-rail input/output op amps with similar supply current. The low supply current at the bandwidth and noise performance allows for excellent fidelity at a fraction of the usual dissipation in portable audio equipment.
Headphone speaker impedances range from 32Ω to 300Ω; their responsivity, from 80dB to 100dB SPL per 1mW and beyond. As an example, considering a headphone speaker with 90dBSPL per 1mW, it takes 100mW to reach 110dBSPL. With 32Ω, the RMS current is 56mA and voltage 1.8V; with 120Ω, 29mA and 3.5V.
Given a 3.3V supply and the output of one LTC6261 amplifier there may not be sufficient drive capability to yield 100mW. However, the combination of two 180 degree phased amplifiers is enough to provide the necessary drive voltage or current to reach upwards of 100mW. Duplication of this bridge drive circuit enables power to both left and right sides.
The LTC6263 provides four amplifiers in one small package. Data from a two-amplifier LTC6262 driving what could be Left or Right is shown below. Basic current consumption of the two amplifiers, with as much as 1VP-P input but no load, is 500µA.
The circuit consists of first an inverting gain stage with closed loop gain = 3, and a subsequent inverting stage. The combination of inverting stages produces a singleended input to differential output gain of 6. With 1VP-P single-ended input, the output is 6VP-P differential, or 3V max (2.1V RMS). With 100Ω, 1V leads to 45mW delivered power.
Despite the low quiescent current, this driver delivers low distortion to a headphone load. At high enough amplitude, distortion increases dramatically as the op amp output clips. Clipping occurs sooner with more loading as the output transistors start to run out of current gain.
Tools & Simulations
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