CN111082800A - I/V converter - Google Patents

At present, digital audio equipment is provided with a DAC chip for converting a digital signal into an analog signal, the analog output signal of the chip is generally of two types, one is a voltage output type, the other is a current output type, a signal source adopted by the circuit is the latter, namely, a current output type, and because the current output type DAC has various types, the circuit only takes a PCM1794A chip of Texas instruments BB company in America as an example of the DAC, and the prior art of a typical I/V converter circuit cascaded with the current output type DAC is shown in the attached

1 of the specification, and as the circuit belongs to a voltage type device, and the PCM1794A chip outputs a current signal, the output stage of the PCM17 1794A chip is more suitable for I/V conversion by using a current input cascaded circuit. In addition, because the inherent high open-loop gain of the operational amplifier makes the operational amplifier be in a large loop depth negative feedback state after being closed, transient intermodulation distortion and phase shift distortion are easy to generate, and because the quiescent current of the operational amplifier tail tube is designed to be very small, the output stage will generate switching distortion and cause recess distortion and crossover distortion, so that the tone quality is deteriorated, particularly represented as sound hair, hardness, harsh and dry.

The technical scheme adopted by the I/V converter for solving the technical problem is as follows: the circuit is provided with discrete components, a large loop negative feedback circuit is eliminated, a common base circuit is directly used for replacing a differential input circuit from an input stage, and the emitter of the common base circuit is directly coupled with the output end of PCM1794A signal current for current transmission, so that the circuit of the invention: q₁、Q₂、Q₄、Q₅And Q₈、Q₁₀Being a cascode circuit, Q₇、Q₉Being a proportional current source with current mode, R₁、R₂Is an I/V conversion resistor. The circuit will Q₁、Q₂(Q₄、Q₅) The collectors of the two transistors are in short circuit, and then the opposite-phase (in-phase) currents output by

18 and 26 (

17 and 25) of the PCM1794A are combined into one, so that the lower circuit is simplified.

The circuit operating principle of the I/V converter of the present invention is as follows: differential currents are output from

18, 26, 17 and 25 of DAC current output pins of the PCM1794A chip. When positive half-cycle signals are output, negative signal currents are output from the terminals of the

18 and 26, and the signal currents enter Q₁、Q₂Emitter re-pass Q₁、Q₂The collector electrodes are connected in parallel and then enter R₅Due to R₅Is constantly pressurized by the luminous tube D1, so that the decrement of the signal current flowing into R5 is equal to the decrement of the signal current flowing through R₄Increment of signal current, set R₄＝VR₁When the 18 pin and 26 pin terminals output negative signal current (I)_OUTR-)+(I_OUTL-)＝-I_R5＝I_R4＝I_VR1＝I_C9(ii) a At the same time, the terminals of the

17 and 25 output positive signal current through Q₄、Q₅Emitter re-pass Q₄、Q₅The parallel collector electrodes enter into R₆Due to R₆Quilt luminous tube D₁Constant pressure, so flows into R₆Is equal to the current through Q₁₀Decrement of collector signal current, so that the I/V conversion formula of the positive half cycle signal is V₀＝I_C9·R₁。

When the negative half-cycle signal is output, the terminals of the

18 and 26 output positive signal current, and the signal current enters Q₁、Q₂Emitter re-pass Q₁、Q₂The parallel collector electrodes enter into R₅Due to R₅Quilt luminous tube D₁At constant pressure, thus flowing into R₅Is equal to the current flowing through R₄Decrease of signal current of (1), set R₄＝VR₁When the terminals of the 18 pins and the 26 pins output positive signal current (I)_OUTR-)+(I_OUTL-)＝I_R5＝-I_R4＝-I_VR1＝-I_C9(ii) a Meanwhile, the terminals of the

17 and 25 output negative signal current through Q₄、Q₅Emitter re-pass Q₄、Q₅The collector electrodes are connected in parallel and then enter R₆Due to R₆Quilt luminous tube D₁Constant pressure, so flows into R₆Is reduced by an amount equal to that of the signal current flowing through Q₁₀The increase of collector signal current, and the I/V conversion of its negative half-cycle signal is-V₀＝-I_C10·R₂。

FIG. 1 is a block diagram of a L, R dual channel output circuit of a typical I/V converter of the prior art, commonly used by manufacturers of products, consisting of PCM1794A and an operational amplifier, available from Texas instruments BB, Inc., USA, from which the output current of PCM1794A passes through the inverting input of the operational amplifier and utilizes the negative feedback resistor R connected to the operational amplifier_fWhen I/V conversion is performed, it is obvious that the operational amplifier operates in a common emitter amplification state from the first differential stage at the inverting input terminal, wherein PCM1794A is a chip for converting digital signals into analog signals, which is abbreviated as DAC, according to the introduction of PCM1794A by BB company, when a single PCM1794A is used in dual channels, its

17 and 18 are R channel current outputs, and its

25 and 26 are L channel current outputs. If 2 PCM1794A are used, one of the two is used as L channel, at this time,

17 and 25 are in-phase current output, and

18 and 26 are in reverse phase current output; when the other chip is used as an R channel,

17 and 25 are in-phase current output, and

18 and 26 are in reverse phase current output.

FIG. 2 shows a circuit diagram of an I/V converter according to the invention, when the converter is used as a power supplyWhen the single-chip PCM1794A is used as the connection mode of single-channel I/V conversion circuit,

18 and 26 of its output terminal pass through the triode Q₁、Q₂Emitter, collector and resistor R₅Is connected to R at one end₅The other end is connected with a negative power supply V-, and a resistor R₅One end of which passes through a triode Q₈Emitter and collector of the triode Q₇The collector is in short circuit with the base and then passes through Q₇Emitter and resistor R of₄One end of the triode is connected with the positive power supply V + of the R4 and the other end of the triode Q₇Base and Q of₉Is connected to the base of a potentiometer VR₁One end is connected with a positive power supply V +, VR₁The other end is connected with Q₉Through the collector and the resistor R₁Is connected to R at one end₁The other end of (3) is grounded, and a resistor R₁One end is respectively connected with a resistor R₇One end of triode Q₁₁Collector and triode Q₁₂Base electrode, resistor R₇The other end is respectively connected with a triode Q₁₁Base, adjustable potentiometer VR₂One terminal, triode Q₁₂Collector connected with positive power supply V + emitter connected with resistor R₈To V₀A terminal;

17 and 25 of DAC output end of PCM1794A chip pass through triode Q₄、Q₅Emitter, collector and resistor R₆One end is connected with R₆The other end is connected with a negative power supply V-, and a resistor R₆One end of the triode Q is connected₁₀The emitter of the transistor is connected with a resistor R through a collector₂One end is connected with R₂The other end is grounded, and a resistor R₂One end is respectively connected with an adjustable potentiometer VR₂One end of triode Q₁₁Emitter, triode Q₁₃Base electrode, V₀Terminal connection resistor R₉One end R₉The other end is connected with a triode Q₁₃Emitter, Q₁₃The collector is connected with a negative power supply V-. Triode Q₃Emitter ground Q₃After the base electrode and the collector electrode are in short circuit with the triode Q₁、Q₂、Q₄、Q₅Is connected with one end of a resistor R3, and a triode Q₆After the collector and the base are short-circuited, the collector and the base are connected with a resistor R₃The other end is connected with a triode Q₈、Q₁₀Base electrode ofConnected, triode Q₆Emitter and luminous tube D₁Anode connected, luminescent tube D₁The negative pole is connected with a negative power supply V-.