FIG. 1 is a circuit diagram illustrating a related transmitter having a push-pull structure. FIG. 1 illustrates a circuit of the transmitter, which has a double stack structure driven in a push-pull manner.
Referring to FIG. 1, an MS1, which is a PMOS transistor, and an MS2, which is an NMOS transistor, are used as constant current sources. PMOS transistors MP1 and MP2, and NMOS transistors MN1 and MN2 are switching elements to determine the direction of current flow. The MP2 and MN1 are simultaneously turned on, and the MP1 and MN2 are simultaneously turned on, thereby supplying a constant current to a positive node 1 and a negative node 2.
A constant voltage is applied across a resistor Rterm by the constant current supplied to the positive node 1 and negative node 2. The constant voltage applied across the resistor Rterm is detected by a comparator 3. Meanwhile, the related transmitter additionally includes a common-mode feedback circuit to fix a common-mode voltage at a voltage vcom because the common-mode noise at the positive node 1 and negative node 2 is high.
The above-mentioned circuit of the related transmitter has three problems as follows. First, although voltage drop occurs at the MS1, MP2, resistor Rterm, resistor Rseri, MN2, and MS2 when the voltage at the positive node 1 is driven to be increased to a level higher than the voltage vcom, high voltage drop occurs at the resistive elements, namely, the MP2, Rterm, Rseri, and MN2 when the driving is achieved using a large current. For this reason, the voltage to be secured in the MS1 and MS2 becomes insufficient. As a result, it is difficult for the MS1 and MS2 to be driven as constant current sources. When the MP1 and MP2 cannot operate as large current sources, it is impossible to supply a large current to a capacitive load, namely, a capacitor Cload. When this happens, it may be impossible to achieve high-speed data transmission.
Second, a lot of time is needed for phase change of the voltage applied across the resistor Rterm when a large capacitive load, for example, the capacitor Cload, is driven. For this reason, it is impossible to charge the capacitor Cload to a normal voltage level in a high-speed operation. To solve this problem, it is necessary to supply a large current to the positive node 1 and negative node 2 upon changing the phase of the above-mentioned voltage. However, the amount of current is limited by the constant current sources. As a result, there is a problem in that it is difficult to achieve rapid phase change at the positive node 1 and negative node 2.
Third, it may be necessary to add the common-mode feedback circuit because the noise at the voltage vcom is high due to high output resistance of the positive node 1 and negative node 2.