Abstract: This article introduced one kind new has the digit volume control function audio frequency power amplifier principle of work and the design method, and will design applies in the actual electric circuit, has obtained the very good effect. Uses the pressed key type volume controller ease of operation, reduced largely with this related software cost, the application prospect has been broad.
Key word: Volume electricity harmonious; Numerical control electric circuit; Resistance network; SPECTRE simulation
The audio frequency power amplifier electric circuit must involve in the practical application to the volume electricity harmonious question, but controls the audio frequency amplification electric circuit’s volume through the adjustment amplifying circuit’s gain is quite effective, at present applies quite many have the following 2 kinds to realize the method: 1) the analogue control method, this method is generally lays aside the resistor on the kneading board, changes the amplifier through manual regulation its size to increase. This method’s shortcoming is the faying surface easy to wear, causes the bad contact, brings the quite big noise; Resistance regulation size not easy accuracy control; Because is the external connection resistance, needs the long lead wire, also will bring the sound the crosstalk. 2) transforms and the electric current voltage variator through the D /A realizes, this method resistance mediation is quite smooth, but does not conform to person’s sense of hearing custom, the gain size is not according to the index movement, therefore also has the volume control not smooth shortcoming. This design uses the numerical control method to realize volume harmonious, the control circuit integration in the tone circuit, will have saved electric circuit’s area directly, moreover may also the accuracy control volume.
1 principle of work and circuit design
This method is and realizes the volume adjustment from this with the rheostat control audio frequency amplification electric circuit’s gain, but the rheostat adjusts by the digital circuit.
The power amplifier gain is by the feedback resistance and the input resistance ratio decision (e.g. Figure 1). Therefore, must adjust the size which the power amplifier increases, only need adjust two resistance Rf and the Ri ratio may. 
Figure 1 audio frequency amplification electric circuit
Resistance regulation’s scale and the scale changes the size may determine according to the actual need that the principle is consistent. In we design, the volume carries on the adjustment between - 33dB ~12dB, each time adjusts 3dB, divides 16 files. Must realize this point to have 17 corresponding resistance composition resistance networks, the simple schematic diagram like chart 2. like this, selects the appropriate resistance value, might and controls the volume using controlling switch S-33~S12. 
Figure 2 realizes the volume control resistance network schematic diagram
1. 1 resistance network design
In the actual circuit design, voltage-division resistance’s selection is a design key. Human’s sense of hearing response along with the acoustic intensity change is not linear, but is approaches to the numerical expression, when the human felt when the sound increases 1 time, in fact the acoustic intensity already increased 10 times. Therefore cannot the simple use equivalent method select in here, but is the use decibel (dB) counts the value. Below gives the resistance selection method, here supposes the total resistance is R. 
Summarizes the above rule, may obtain passes a formula: 
This may act according to the actual technological conditions and the accuracy requirement chooses the appropriate resistance value. What we use in the design pours into the resistance, the final adjustment curacy error will control in 10%.
1. 2 resistance control electric circuit’s principle of work and ASIC design
Resistance Ri and the Rf control circuit’s structure mainly divides into POWER the RESET electric circuit, the counting circuit, the decoding circuit, the analog switch electric circuit and so on several parts. On the electricity will later use to the electric capacity charging and discharging control, by POWER RESET electric circuit when the power source will achieve some level will produce a signal to reposition for the following D trigger, and will cause the gain replacement is 1 finally, the corresponding volume will be 0dB, this time will output and the input peak-to-peak value is the same. Volume enlargement and reduces is controls by 2 input signal CLOCK and UP/DOWN, CLOCK continuously for when low level or high level increases does not change, the volume maintains the original value; When CLOCK rise along time gathers the UP/DOWN condition, if for high, the counter “adds 1″, carries in S-33~S12 through the following 4 -16 decoder selection analog switch’s control, S-33~S12 connects the different resistance number separately the resistance R1 ~R17 vertex. Feedback resistance Rf increases, negative input resistance Ri to reduce, the amplifier increases increases, volume fill-out. The volume reduces control method just right and this opposite.
In electric circuit actual design, after replacement the COUNTER module in CLOCK, under the UP/DOWN signal’s control, realizes “adds 1″ or “reduces 1″ the function to provide to the following decoding circuit, and after the behind 4 - 16 decodings is 16 switches provides the input control, the analog switch (S-33 ~S12) is transmits the gate constitution by 16. In order to reduce under the shut-down condition the useless power dissipation, in the electric circuit has also joined the SHARTDOWN module. Designs by the above principle realizes the electric circuit specifically, electric circuit’s function diagram as shown in Figure 3: 
Figure 3 resistance control electric circuit’s function diagram
2 simulations and practical application
To has used the digital volume tuning circuit’s audio frequency power amplifier simulation using the SPECTRE simulator, joins a peak-to-peak value is 1 V, the frequency is the 1kHz sine drive signal, and adjusts CLOCK and the UP /DOWN, obtains curve which shown in Figure 4.
Figure 4 audio frequency power amplifier simulation result
The curve which obtains from the simulation result we may see, this kind of belt digit electricity harmonious tone circuit after replacement can realize the volume 16 grade of index adjustment, moreover, if examines in the process which in the process which the volume increased already the maximizing or reduced to examine already achieved the minimum value, could maintain. In the actual design controls the SHART2DOWN electric current in 0. 3μA within. Uses this methods design the volume adjuster not only to be possible to apply in the audio frequency amplification electric circuit chip design, may also use in other need rheostat’s place.
