Opamps

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ADJUSTING DC-OFFSET VOLTAGES IN MULTI-STAGE AMPLIFIERS

Figure 17

In single stage amplifiers, the opamp´s DC offset voltage is only a few milliamps and is rarely a problem. In multistage amplifiers, DC offsets may be amplified by successive stages until the idle voltage at the output of the final stage reaches several volts, although the overall gain the system may not be very high. Jan Meier experienced this situation while building and testing a headphone amplifier:

Referring to figure 17a, the non-inverted input of an opamp wired as a voltage follower requires a small input bias current (i+) that, since it flows through the resistor R1, generates a non-zero voltage V+ = (i+)*R1 at the input. Typical values for i+ are 1uA to 2uA (LM6171/LM6181/LTC1206) for bipolar-input, or 1 to 50 pA (OPA627/OPA604) for FET-input amplifiers. With a R1 of 100K, V+ (and thus Vout) can have values up to 200 millivolts!

In figure 17b, a feedback loop is added that amplifies V+ by a factor (R3+R2)/R2. It is not unusual for a headphone amplifier to have a gain factor of around 5. This will, however, also amplify V+ for a Vout of up to 1000mV, which can damage headphones especially low impedance headphones. Fortunately the inverted input also generates a bias current (i-) that generates a DC-voltage (V-) at the inverted terminal and thus counteracts the effects of V+. The effective resistance to ground seen by the inverted input is the value of R2 and R3 in parallel which equals (R2R3)/(R2+R3). To eliminate the output voltage offset generated by i+, the input voltage Vshould be equal to

V+: (i+)R1 = (i-)*(R2R3)/(R2+R3)

To s e l e c t values for R2 and R3, first take a look at the specifications of the opamp for i+ and i-. Note that they do not have to have the same value. For instance, the LTC1206 has an i+ value of 2uA whereas igoes up to 10uA! By a proper selection of the resistor values, the offset can be strongly reduced. With a headphone amplifier made from a LM6171 opamp and having R1 = 47 kOhm, R2 = 56 kOhm, R3 = 300 kOhm, one channel shows a very good offset of only 20 mV. The other channel came down to a hardly measurable 0.2 mV! The fact that the channels were not equal simply has to do with manufacturing variations in opamps of the same type.

A problem remains with the input stage. If the input potentiometer is directly coupled to the opamp, the value of R1 now changes with the volume control, and a perfect fit of the resistances can not be made. A possible solution is shown in figure 17c. The resistance of the potentiometer no longer has an influence on the DC-resistance of the opamp. Alternatively, if the headphone amplifier has a second stage, the input stage can be decoupled from the second stage as shown in figure 17d. If the output of a first stage is directly coupled to the input of a second stage, the effective value of R1 is zero and a match can not be made. However, you simply can put a resistor between output and input.

Last warning: If the headphone amplifier will also be a preamplifier, any DC-offset at the output will be amplified by the power amp and will be fed into a low resistance loudspeaker. In this situation a few millivolts offset can damage the loudspeaker. To prevent any damage to loudspeakers or to the power amp, always use (decent quality) capacitors at the output of the preamp.

Figure 18a

Multitrack recording allows musicians to record songs in layers. Tracks can be added or overdubbed. Musicians may be positioned far apart from each other or play at different times to isolate their performances for the greatest flexibility in editing. Headphone monitoring is the most common way for musicians to hear each other under these circumstances, and a headphone distribution amplifier is central to this function. Headphone distribution amplifiers can drive several pairs of headphones from a single set of inputs. While it is fairly easy to build one from a power amplifier with a ladder of output resistors, there are advantages to driving each headphone from its own amplifier, such as greater control over gain. The first stage of the basic distribution amplifier shown in figure 18a is a voltage follower that provides impedance buffering and signal inversion for correct phase at the headphone output. The buffer feeds any number of headphone amplifier blocks with their own volume controls.

Figure 18b

As more musicians demand custom mixes, so commercial distribution amplifiers have begun adding mixer features. Figure 18b shows how to convert the input buffer stage of the basic distribution amplifier into a mixer stage. The input buffers (A1 and A2 for the left and right channels) now have a series of 100K summing resistors, one resistor for each stereo or mono input. The level controls for the stereo and mono inputs are balance-volume and pan-volume sets of pots. To move the balance or panning characteristic closer to the ends of the pot rotation, decrease the value of Ri.

Figure 18c

A full-featured headphone distribution amplifier will have limiters and possibly equalization stages for each headphone output. An acoustic simulation network, equalizer and/or limiter can be placed between the buffer and headphone amplifier blocks (see Designing a Limiter for Headphone Amplifiers for information about limiters and Designing a Pocket Equalizer for Headphones for equalization schematics). To increase the drive capability of an amp block, add a current buffer output stage (after any active EQ stages).

Keywords : Opamp, Operational Amplifier, Headphone, P-amp, Configuring, Opamps, For, Voltage, Gain
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Writer : delon  |
27 Feb 2011 Mon
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