Full scheme of controlling DC Motor depends upon

(1) DC motor voltage & wattages -
Low voltage 12V-48 V or high voltage - 220-400V
POwer fraction HP to 1,3,5 HP
(2)Type of control -
(a) without or without direction control
(forward/reverse)
(b) Accuracy of control
(c) Time Response Desired

DC motor control scheme will be quite different for combination of above parameter.

Like for low power, low volt DC motor a single transistor circuiit(Bipolar can be used but now a days MOSFET /IGBT are used) with volt(& hence RPM) control is achieved using driving base through a single PWM
But if Direction control is also required then 4 trans
bridge is requirede with seperate PWM signal for each trans need to be generated. Their phase relation , level shifting , dead time complicated the circuit.
For High Voltgage , High Power DC motor generally controlled thyrisotor(SCR) bridges are used with phase conrolled firing of SCR to achieve desired output DC voltage. Phase angle of these SCR is derived using armature voltgae of tacho for speed feedback.
Plesase specify your appl and other details then a pinpointed solution can be suggested.

Anil