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