The level converted high and low group sine wave signals are combined by {HEAD ~ a mixer} comprising {TAIL ~ buffer amplifiers}, resistors and a transistor, thereby obtaining a DTMF signal 
The level converted high and low group sine wave signals are combined by { HEAD ~ a mixer } comprising { TAIL ~ buffer amplifiers } , resistors and a transistor , thereby obtaining a DTMF signal
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH